REPRODUCTIVE SYSTEM
Male Reproductive System
Testes - contain spermatogenic stem cells that continuously divide to produce new generations of cells.
Stem cells → spermatozoa/cell
Epididymis - storage and maturation
Testes → epididymis → ductus/ vas deferens → penile urethra
Accessory glands - prostate gland, seminal vesicles, and bulourethral - of the male reproductive system
Scrotum - the pair of testes is located here
The temperature of the testes is about 2 to 3 degrees lower than normal body temperature.
Spermatogenesis - sperm production
Pampiniform plexus - where the testicular arteries that descend into the scrotum are surrounded by a complex plexus of veins that ascend from the testes
Countercurrent heat-exchange mechanism - where arterial blood is cooled by venous blood before it enters the testes, helping maintain a lower temperature in the testes
Testes - a thick connective tissue capsule
Tunica albuginea - surrounds each testis
The tunica albuginea thickens and extends inwards into each testis to form the medisstinum testis.
Septum - a thin connective tissue that extends from the mediastinum testis and subdivides each testis into about 250 incomplete compartments
Testicular lobules - the 250 incomplete compartments that contain one to four coiled seminiferous tubules
Seminiferous tubule - is lined by germinal epithelium, which contains proliferating spermatogenic (germ) cells and non-proliferating supporting (sustentacular) or Sertoli cells.
Surrounding each seminiferous tubule are fibroblasts,muscle-like cells, nerves, blood vessels, and lymphatic vessels
Interstitial cells (of Leydig) are clusters of epithelial cells; these cells secrete steroids and produce the male sex hormone testosterone, which is involved in forming sperm.
Spermatogenesis - process of sperm formation
It includes mitotic divisions of spermatogenic cells, which produce replacement stem cells and other spermatogenic cells that eventually give rise to primary and secondary spermatocytes.
Meiotic divisions reduce the number of chromosomes and the amount of DNA
Spermatids - division of secondary spermatocytes produces cells that contain 23 single chromosomes (22 + X or 22 Y)
Spermiogenesis - spermatids do not undergo any further divisions, but instead are transformed into sperm by this process
Intercellular bridges - they hold the spermatogenic cells in the germinal epithelium, which differentiate.
The intercellular bridges are broken when the developed spermatids are released into the seminiferous tubules as mature sperm.
Transformation of Spermatids
Spermiogenesis - is a complex morphologic process by which the spherical spermatids are transformed into elongated sperm cells.
The size and shape of the spermatids are altered, and the nuclear chromatin condenses.s
Golgi phase - is a small granule that accumulates in the Golgi apparatus of the spermatid and forms an acrosomal granule within a membrane-bound acrosomal vesicle.e
Acrosomal phase - both the acrosomal vesicle and acrosomal granule spread over the condensing spermatid nucleus, forming the anterior tip of the spermatid as an acrosome
Acrosome - it functions as a specialized type of lysosome and contains several hydrolytic enzymes, such as hyaluronidase and protease with trypsin-like activity, that assist the sperm in penetrating the cells (corona radiata) and the membrane (zona pellucida) that surround the ovulated oocyte
Maturation phases - the plasma membrane moves posteriorly from the nucleus to cover the developing flagellum (sperm tail)
The mitochondria migrate to and form a tight sheath around the middle piece of the flagellum.
Final maturation phase - characterized by the shedding of the excess or residual cytoplasm of the spermatid and release of the sperm cell into the lumen of the seminiferous tubule
Sertoli cells then phagocytose the residual cytoplasm
The mature sperm cell is composed of a head and an acrosome that surrounds the anterior portion of the nucleus, a neck, a middle piece characterized by the presence of a compact mitochondrial sheath, and a main or principal piece
Excurrent Ducts
Seminiferous tubules → intertesticular excurrent ducts →testis → epididymis
Excurrent ducts consist of the:
Straight tubules (tubuli recti)
Rete testis - the epithelial-lined spaces in the mediastinum testis
Rete tesitis → 12 short tubules (ductuli efferntes)
Ductuli efferentes (efferent ducts) - conduct sperm from the rete testis to the initial segment or the head of the epididymis
Ductus epididymis - the extratesticular duct that conducts the sperm to the penile urethra
Continuous with the ductus (vas) deferens and the ejaculatory ducts in the prostate gland
During sexual excitation and ejaculation, strong contractions of the smooth muscle that surrounds the ductus epididymis expel the sperm
Functional Correlations: Testes
Spermatogonia - the function of the testes is to produce both sperm and testosterone
Testosterone - an essential hormone for the development and maintenance of male sexual characteristics and normal functioning of the accessory reproductive glands
The spermatogenic cells in the seminiferous tubules divide, differentiate, and produce sperm by a process called spermatogenesis
Spermatogenesis involves the following:
Mitotic divisions of spermatogonia to form stem cells
Formation of primary and secondary spermatocytes from spermatogenic cells
Meiotic divisions of primary and secondary spermatocytes reduce the somatic chromosome numbers by one half, and form spermatids, which are germ cells with only 23 single chromosomes (22+X or 22+Y)
Morphologic transformation of spermatids into mature sperm by a process called spermiogenesis
Sertoli cells - the supportive cells of the testes that are located among the spermatogenic cells in the seminiferous tubules
They perform the following functions:
Physical support, protection, and nutrition of the developing sperm (spermatids)
Phagocytosis of excess cytoplasm (residual bodies) from the developing spermatids
Release of mature sperm, called spermiation, into the lumen of the seminiferous tubules
Secretion of fructose-rich testicular fluid for nourishment and transport of sperm to the excurrent ducts
Production and release of androgen-binding protein (ANP) that binds to and increases the concentration of testosterone in the lumen of the seminiferous tubules, which is necessary for spermatogenesis
ABP secretion - under the control of follicle-stimulating hormone (FSH) from the pituitary gland
Secretion of the hormone inhibin, which suppresses the release of FSH from the pituitary gland
Production and release of the anti-mullerian hormone, also called mullerian-inhibiting hormone, suppresses the development of mullerian ducts in the male and inhibits the development of female reproductive organs.s
Blood-Tests Barrier
The adjacent cytoplasm of Sertoli cells is joined by tight junctions, producing a blood-testis barrier that subdivides each seminiferous tubule into a basal compartment and an adluminal compartment.
This segregates the spermatogonia from all successive stages of spermatogenesis in the adluminal compartment and excludes the plasma proteins and bloodborne antibodies from the lumen of the seminiferous tubules.
This barrier prevents an autoimmune response to the individual’s own sperm, antibody formation, and eventual induction of sterility.
It also keeps harmful substances in the blood from entering the developing germinal epithelium.
Testis (Sectional View)
Testis - enclosed in a thick connective tissue capsule called tunica albuginea.
Internal to which is a vascular layer of loose connective tissue called tunica vasculosa
The connective tissue extends inwards from the tunica vasculosa → testis to form the interstitial connective tissue → binding and supporting the seminiferous tubules.
Extending from the medistinum testis → tunica albuginea → septum → that divides the testis into compartments called lobules
Within each lobule are found one to four seminiferous tubules
Septa - are not solid, and there is intercommunication between lobules
Located in interstitial connective tissue and clusters of epithelial interstitial cells (of Leydig)
Interstitial cells - are the endocrine cells of the testis and secrete the male sex hormone testosterone into the bloodstream
Seminiferous tubules - are long, convoluted tubules in the testis that are normally observed cut in transverse, longitudinal, or tangential planes of section
They are lined with a stratified epithelium called the germinal epithelium
Which contains two cell types, the spermatogenic cells that produce sperm and the supportive Sertoli cells that nourish the developing sperm
It rests on the basement membrane of the seminiferous tubules and its cells
Seminiferous Tubules, straight tubules, rete testis, and Ductuli Efferentes (Efferent Ductules)
In the posterior region of the testis, the tunica albuginea extends into the testis interior as the mediastinum testis.
The plane of section passes through the seminiferous tubules → the connective tissue and blood vessels of the mediastinum testis, → the excretory ducts (ductuli efferentes)
A few seminiferous tubules - are visible on the left side; they are lined with spermatogenic epithelium and sustentacular (Sertoli) cells.
The interstitial connective tissue is continuous with the medistinum testis and contains steroid (testosterone)-producing interstitial cells (of Leydig)
In the mediastinum testis - the seminiferous tubules terminate in the straight tubules.
Straight tubules - are short, narrow ducts lined with cuboidal or low columnar epithelium that are devoid of spermatogenic cells.
Mediastinum testis → straight tubules → rete testis of the mediastinum testis
Rete testis - is an irregular, anastomosing network of tubules with wide lumina lined by a simple squamous to low cuboidal or low columnar epithelium; it becomes wider near the ductuli efferents, into which the rete testis empties.
Ductuli efferentes - connect the rete testis with the epididymis
Epithelium of the ductuli efferentes - consists of groups of tall columnar cells that alternate with groups of shorter cuboidal cells
The lumina of the efferents are uneven
The tall cells in the ductuli efferentes exhibit cilia, and the cuboidal cells exhibit microvilli.
Hormones of Male Reproductive Organs
Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) - produced by gonadotrophs in the adenohypophysis of the pituitary gland.
LH binds to receptors on interstitial cells of Leydig. And stimulates them to synthesize the hormone testosterone.
FSH stimulates Sertoli cells to synthesize and release androgen-binding protein (ABP) into the seminiferous tubules, which stimulates spermatogenesis
Inhibin - a hormone secreted by the Sertoli cells, has an inhibitory effect on the pituitary gland and suppresses or inhibits additional production of FSH.
Primate Testis: Spermatogenesis in Seminiferous Tubule (Transverse Section)
Each seminiferous tubule is surrounded by an outer layer of connective tissue, fibroblasts, and an inner basement membrane.
Between the seminiferous tubules - are interstitial tissue with fibroblasts, blood vessels, nerves, lymphatics, and interstitial cells (of Leydig)
Germinal epithelium of the seminiferous tubule consists of supporting or Sertoli cells and spermatogenic cells.
Sertoli cells - are slender, elongated, with irregular outlines that extend from the basement membrane to the lumen of the seminiferous tubule.
The nuclei of Sertoli cells - are ovoid or elongated and contain fine, sparse chromatin.
Spermatogonia - immature spermatogenic cells are adjacent to the basement membrane of two of the seminiferous tubules
They are divided mitotically to produce several generations of cells. There are three types of spermatogonia.
Pale type A spermatogonia - have a light-staining cytoplasm and a round or ovoid nucleus with pale, finely granular chromatin.
Serves as stem cells for the germinal epithelium and gives rise to other types
Dark type A spermatogonia - appear similar but with darker chromatin
Type B spermatogonia
They produce primary spermatocytes - which are the largest germ cells in the seminiferous tubules and occupy the middle region of the germinal epithelium.
The secondary spermatocytes undergo a second meiotic division shortly after formation and are not frequently seen in the seminiferous tubules.
The second meiotic division produces spermatids that are smaller than the primary or secondary spermatocytes.
Primary testis: Stages of Spermatogenesis
Three stages of spermatogenesis:
Primary spermatocytes → form secondary spermatocytes →undergo rapid meiotic division to form spermatids → embedded deep in the Sertoli cell cytoplasm (adjacent to the basement membrane are the type A spermatogonia
Spermatids near the lumen of the seminiferous tubule before their release → visible are round spermatids and primary spermatocytes close to Sertoli cells → near the base are the spermatogonia.
Mature sperm have been released (spermiation) → to the seminiferous tubule, and the germinal epithelium contains only spermatids → primary spermatocytes, spermatogonia, and supporting Sertoli cells.
Testis: Seminiferous Tubules (Transverse Section)
Seminiferous tubule and parts of the adjacent seminiferous tubules
Dark type A and the pale type B spermatogonia are located at the base of the tubule
The primary spermatocytes and spermatids in different stages of maturation are embedded in the germinal epithelium tubules
The supportive Sertoli cells are located throughout the germinal epithelium
Each seminiferous tubule is surrounded by a fibromuscular interstitial connective tissue
Ductuli Efferentes and Tubules of Ductus Epididymis
Ductuli efferentes or efferent ductules emerge from the mediastinum on the posterosuperior surface of the testis and connect the rete testis with the ductus epididymis.
The ductuli efferentes are located in the connective tissue and form a portion of the head of the epididymis.
The lumen of the ductuli efferentes exhibits an irregular contour because the lining epithelium consists of simple alternating groups of tall ciliated and shorter non-ciliated cells.
Located under the basement membrane is a thin layer of connective tissue containing a thin smooth muscle layer
The ductus epididymis is lined with pseudostratified columnar epithelium of the
It is a long, convoluted tubule surrounded by connective tissue and a thin smooth muscle layer.
A section through the ductus epididymis shows both cross sections and longitudinal sections.s
Some parts of the ductus contain mature sperm.m
The pseudostratified columnar epithelium consists of tall columnar principal cells with long, nonmotile stereocilia and small basal cells.
Tubules of Ductus Epididymis (Transverse Section)
The tubules of the ductus are lined with pseudostratified epithelium
Principal cells - are tall columnar epithelium cells and are lined with stereocilia, the long, branching microvilli
Basal cells - are small and spherical and situated near the base of the epithelium
A thin layer of smooth muscle surrounds each tubule. Adjacent to the smooth muscle layer are cells and fibers of the connective tissue
Ductus (Vas) Deferens (Transverse Section)
Ductus (Vas) Deferens - exhibits a narrow and irregular lumen with longitudinal mucosal folds; a thin mucosa, thick muscularis, and an adventitia.
The lumen of the ductus deferens is lined by pseudostratified columnar epithelium with stereocilia.
It is somewhat lower than in the ductus epididymis
The underlying thin lamina propria consists of compact collagen fibers and a fine network of elastic fibers
The thick muscularis consists of three smooth muscle layers: a thinner inner longitudinal layer, a thick middle circular layer, and a thinner outer longitudinal layer
The muscularis is surrounded by adventitia, where abundant blood vessels, benule, arterioles, and nerves are found
The adventitia of the ductus deferens merges with the connective tissue of the spermatic cord
Ampulla of the Ductus (Vas) Deferens (Transverse Section)
Ampulla mainly differs from the ductus deferens in the structure of its mucosa
Lumen of the ampulla - is larger than that of the ductus deferens
The mucosa also exhibits numerous irregular, branching mucosal folds and deep glandular diverticula or crypts located between the folds and extend to the surrounding muscle layer
The secretory epithelium that lines the lumen and the glandular diverticula is simple columnar or cuboidal, and below the epithelium is the lamina propria
The smooth muscle layers in the muscularis are similar to those in the ductus deferens
Consisting of a thin inner longitudinal muscle layer
A thick middle circular muscle layer
A thin outer longitudinal muscle layer surrounds the ampulla; the connective tissue adventitia.
Summary: The Male Reproductive System: Composition
Consists of two testes that contain spermatogenic cells, which produce sperm
Numerous excurrent ducts move sperm for storage and maturation into the ductus epididymis
During ejaculation, sperm leave the system via the ductus (vas) deferens and the penile urethra
Accessory glands include the prostate, seminal vesicles, and bulbourethral glands
Scrotum
Testes are located outside the body in the scrotum, whose temperature is 2 to 3 degrees Celsius lower than the body temperature.
Lower temperature in the scrotum is a result of sweat evaporation and the pampiniform plexus.
A countercurrent heat-exchange mechanism in the veins cools the material blood as it enters the testis.
Testes
Thick connective tissue, the tunica albuginea, surrounds each testis and forms the mediastinum testis.
Thin connective tissue septa from the mediastinum testis separate the testis into testicular lobules.
Testicular lobules contain coiled seminiferous tubules that are lined by germinal epithelium.
Germinal epithelium contains spermatogenic cells and Sertoli (supportive) cells.s
Between the seminiferous tubules are testosterone-secreting interstitial cells (of Leydig)
Spermatogenesis
Include mitotic divisions of spermatogenic cells to form type A stem cells
Spermatogenic cells type B give rise to primary spermatocytes, the largest cells in the tubules
Primary spermatocytes give rise to smaller secondary spermatocytes
Meiotic divisions of primary and secondary spermatocytes reduce the number of chromosomes
Secondary spermatocytes divide to form spermatids
Spermatids do not divide to form spermatids
Spermatids do not divide and contain 23 chromosomes (22 + X or 22 Y)
Developing sperm are connected by intercellular bridges until they are released as mature sperm tubules
Spermiogenesis
Morphologic transformation of spermatid into sperm
The size and shape of the spermatid are altered with the condensation of nuclear chromatin
On the anterior side, acrosome granules in the vesicle spread over the condensing nucleus as the acrosome
The acrosome contains hydrolytic enzymes needed to penetrate the cells that surround the oocyte
On the posterior side, the flagellum (tail) forms with mitochondria aggregating at the middle piece of the sperm
Residual cytoplasm shed from spermatids and phagocytosed by Sertoli cells
Mature sperm consists of the head, neck, middle piece, and principal piece
Excurrent Ducts
Released sperm pass through the straight tubules and the rete testis to the ductuli efferentes
Ductuli efferentes emerge from the mediastinum and conduct sperm to the head of the ductus epididymis.
Epithelium of the ductile efferentes is uneven owing to ciliated and nonciliated cells in the lumina.
Cilia in the ductus efferentis move sperm and fluid from the seminiferous tubules to the ductus epididymis.s
Nonciliated cells absorb much of the testicular fluid as it passes to the ductus epididymis.s
The ductus epididymis is continuous with the ductus (vas) deferens that conducts sperm to the penile urethra.
Smooth muscles around the ductuli efferentes, the ductus epididymis, and the vas deferens contract to move sperm
Pseudostratified epithelium with principal and basal cell lines,ductuli efferentes, and epididymi.s
Stereocilia line the surface of cells in the ductus epididymis and vas deferens
Stereocilia absorb testicular fluid, and the principal cells phagocytose residual cytoplasm.m
Principal cells in the ductus epididymis also produce glycoproteins that inhibit sperm capacitation.
Sertoli Cells
Physical support, protection, nutrition, and release of mature sperm into the tubules
Phagocytosis of the residual cytoplasm of spermatids
Secretion of ABP concentrates testosterone in the tubules and testicular fluid for sperm transport
Secretion of hormone inhibin and anti-mullerian hormone
Blood-Testis Barrier
Formed by tight junctions of adjacent Sertoli cells
Separate the seminiferous tubules into the basal and adluminal compartments
Protects developing sperm from autoimmune response and harmful materials
Male Hormones
Spermatogenesis is dependent on LH and FSH hormones produced by the pituitary gland.
LH binds to receptors on interstitial cells and stimulates testosterone secretion.
FAH stimulates Sertoli cells to produce ABP into the seminiferous tubules to bind testosterone
Testosterone in the seminiferous tubules is vital for spermatogenesis and accessory gland function.
Sertoli cells produce inhibin, which inhibits FSH production from the pituitary gland.
.
Accessory Reproductive Glands
Seminal Vesicles, Prostate Gland, Bulboutrethral Glands, and Penis
The accessory glands of the male reproductive system consist of paired seminal vesicles, paired bulbourethral glands, and a single prostate gland
The penis serves as the copulatory organ, and the penile urethra serves as a common passageway for urine or semen
A secretory product mix with sperm and fluid
Seminal vesicle- located posterior to the bladder and superior to the prostate gland.
Ampulla → ejaculatory ducts → prostate gland →PROSTATIC URETHRA
Prostate gland - located inferior to the neck of the bladder
Urethra →bladder → prostate gland → prostatic urethra
Bulbourethral glands - small, pea-sized glands located at the root of the penis and embedded in the skeletal muscles of urogenitdal diaphragm; their excretory ducts terminate in the proximal portion of the penile urethra
Penis - consists of erectile tissues, the paired dorsal corpora cavernosa and a single ventral corpus spongiosum that expands distally into the glans penis
The penile urethra extends through the entire length of the corpus spongiosum, this portion of the penis is also called corpus cavernosum urethrare
Each erectile body in the penis is surrounded by the connective tissue layer tunica albuginea
Blood enters the vascular spaces from the branches of the dorsal artery adn deep arteries of the penis and is drained peripheral veins
Prostate Gland and Prostatic Urethra
Prostatic urethra - the location where the urethra that leaves the bladder and passes through the prostate gland
Transitional epithelium - lines the lumen of the crescent-shaped prostatic urethra.
Prostatic glands - small, branched tubuloacinar
Prostatic concretions - solid secretory aggregations in their acini
On each side of the colliculus seminalis are the prostatic sinuses
Most excretory ducts of the prostatic glands open into the prostatic sinuses
In the middle of the colliculus seminalis is a cul-de-sac called the utricle
It often shows dilation at its distal end before it opens into the prostatic urethra
Two ejaculatory ducts open at the colliculus, one on each side of the utricle
Prostate Gland: Glandular Acini and Prostatic Concretions
The size of the glandular acini in the prostate gland is highly variable
The lumina of the acini are normally wide and typically irregular because of the protrusion of the epithelium-covered connective tissue folds
Some of the glandular acini contain proteinaceous prostatic secretion
Other glandular acini contain spherical prostatic concretions that are formed by concentric layers of condensed prostatic secretion
Prostatic concretions are characteristic features of the prostate gland acini
The number of prostatic concretions increases with the age of the individual, and they may become calcified
Glandular epithelium is usually simple columnar or pseudostratified, and the cells are light-staining. There is considerable variation
In some regions, the epithelium may be squamous or cuboidal
Excretory ducts of the prostatic glands may often resemble the glandular cini
In the terminal portions of the ducts, the epithelium is usually columnar and stains darker before entering the urethra
The fibromuscular stroma is another characteristic feature of the prostate gland; smooth muscle bundles and the connective tissue fibers blend together on the stroma and are distributed throughout the gland
Prostate Gland: Prostatic Glands with Prostatic Concretions
The glandular epithelium also varies from simple cuboidal or columnar to pseudostratified epithelium.
In older individuals, the secretory material of the prostatic glands precipitates to form the characteristic dense-staining prostatic concretion.s
The prostate gland is also characterized by the fibromuscular stroma. a
Seminal Vesicle
Seminal vesicles - are elongated glands located on the posterior side of the bladder
It exhibits highly convoluted and irregular lumina; a cross section through the gland illustrates the complexity of the primary mucosal folds, where they branch into numerous secondary mucosal crypts.
Lamina propria projects into and forms the core of the larger primary folds and the smaller secondary folds, where the fold extends far into the lumen of the seminal vesicle
The glandular epithelium of the seminal vesicles varies in appearance, but is usually low pseudostratified and low columnar or cuboidal.l
The muscularis consists of an inner circular muscle layer and an outer longitudinal muscle layer. This arrangement of the smooth muscles is often difficult to observe because of the complex folding of the mucosa. a
The adventitia surrounds the muscularis and blends with connective tissue.
Bulbourethral Gland
The fibroelastic capsule that surrounds these glands contains connective tissue, smooth muscle fibers, and skeletal muscle fibers in the interlobular connective tissue septum.
The secretory units vary in structure and size and resemble mucous glands
The glands exhibit either acinar secretory units or tubular secretory units
Secretory cells - are cuboidal, low columnar, or squamous and light staining, where the height of the epithelial cells depends on the functional state og the gland
Smaller excretory ducts from the secretory may be lined with secretory cells, whereas the larger excretory ducts exhibit pseudostratified or stratified columnar epithelium
Accessory Male Reproductive Glands
Semen - the product of the secretory products from the seminal vesicles, prostate gland, and bulbourethral glands
It provides the sperm with a liquid transport medium and nutrients
It also neutralizes the acidity of the male urethra and vaginal canal, and activates the sperm after ejaculation
Seminal vesicles - produce a yellowish, viscous fluid that contains a high concentration of sperm-activating chemicals, such as fructose (the main carbohydrate component of semen)
Fructose - is metabolized by sperm and serves as the main energy source for sperm motility; seminal vesicles produce most of the fluid found in semen
Prostate gland - produces a thin, watery, slightly acidic fluid, rich in citric acid, prostatic acid phosphatase, amylase, and prostate-specific antigen (PSA)
Fibronolysin in the fluid liquefies the congealed semen after ejaculation
PSA is very useful for the diagnosis of prostatic cancer because its concentration often increases in the blood during malignancy
Bulbourethral glands - produce a clear, viscid, mucouslike secretion that, during erotic stimulation, is released and serves as a lubricant for the penile urethra
During ejaculation, secretions from the bulbourethral glands precede other components of the semen
Human Penis (Transverse Section)
A cross section of the human penis illustrates the two dorsal corpora cavernosa (singular: corpus cavernosum) and a single ventral corpus spongiosum that form the body of the orga.n
Urethra → penis (in the corpus spongiosum)
Tunica albuginea - a thick connective tissue capsule that surrounds the corpora cavernosa and forms the median septum between the two bodies
Thinner tunica albuginea - with smooth muscle fibers and elastic fibers, surrounds the corpus spongiosum
Deep penile (Buck’s) fascia - loose connective tissue surrounding the corpus spongiosum and corpora cavernosa
Dermis - covers the deep penile (Buck’s) fascia.
Located below the stratified squamous keratinized epithelium of the epidermis
Strands of smooth muscle of the dartos tunica, nerves, sebaceous glands, and peripheral blood vessels are located in the dermis.
Trabeculae with collagenous, elastic, nerve, and smooth muscle fibers surround and form the core of cavernous sinuses (veins) in the corpora cavernosa and corpus spongiosum
The sinuses are lined with endotheloum and receive the boold from the dorsal arteries and deep arteries of the penis.
Deep arteries branch in the corpora cavernosa and form the helicine arteries, which empty directly into the cavernous sinuses
Blood leaving the cavernous sinuses exits mainly through the superficial vein and the deep dorsal vein.
Urethra → base of the penis → epithelium changes to stratified to squamous
Penile Urethra (Transverse Section)
Corpus spongiosim - surrounds the penile urethra and it extends the entire length of the penis.
The lumen of the penile urethra is visible in the transverse section
The lining of this portion of the urethra is a pseudostratified or stratified columnar epithelium
A thin underlying lamina propria merges with the surrounding connective tissue of the corpus spongiosum
Numerous irregular outpockets or urethral lacunae with mucous cells are found in the lumen of the penile urethra
Urethral alcunae - are connected with the branched mucous urethral glands of (Littre) located in the surrounding connective tissue of the corpus spongiosum and found throughout the length of the penile urethra
The corpus spongiosum consists of cavernous sinuses lined by endothelial cells and separated by connective tissue trabeculae that contain smooth muscle fibers and collagen fibers
Numerous blood vessels, arterioles and venules, supply the corpus spongiosum; the internal structure of the corpus spongiosum is similar to that of the corpora cavernosa
Summary of Accessory Reproductive Glands:
Seminal vesicles - located posterior to the bladder and superior to the prostate gland
Excretory ducts join with the ampulla of the vas deferens to form the ejaculatory ducts
Ejaculatory ducts continue through the prostate gland to open into the prostatic urethra
Produce fluid with sperm-activating fructose, the main energy source for sperm motility
Produce most of the fluid found in semen
Prostate Gland - located inferior to the neck of the bladder
The urethra exits the bladder and passes through the prostate as the prostatic urethra
Excretory ducts from prostatic glands enter the prostatic urethra. The transitional epithelium lines the prostatic urethra
Characterized by fibromuscular stroma and prostatic concretions in the glands
Produces watery secretion with numerous chemicals, including prostate-specific antigen
Bulbourethral Glands
Small glands located at root of penis and in skeletal muscle of urogential diaphragm
Excretory ducts enter the proximal part of the penile urethra
Produce a mucous-like secretion that serves as a lubricant for the penile urethra
Penis
Consists of erectile tissue or vascular spaces lined by endothelium
Erectile corpora cavernosa is located on dorsal side and copus spongiusm on ventral side
Tunica albuginea surrounds the erectile bodies
Dorsal artery and deep artery supply the erectile bodies with blood
Female Reproductive System
Overview of the Female Reproductive System
The human female reproductive system consists of paired internal ovaries, paired uterine (fallopian) tubes, and a single uterus.
Inferior to the uterus and separated bby the cervix is the vagina
Mammary glands are associated with the female reproductive system
Human female reproductive organs exhibit cyclical monthly changes in both structure and function (menstrual cycle)
The appearance of the initial menstrual cycle in the maturing individual (menarche)
The cycle becomes irregular and eventually disappears (menopause)
Menstrual cycle is primarily controlled by two hormones secreted by the adenohypophysis of the pituitary gland, follicle-stimulating hormone (FSH) and luteinizing hormone (LH), and by two ovarian steroid hormones, estrogen and progesterone
The release of FSH and LH from the pituitary gland is controlled by releasing factors, hormones secreted by neurons in the hypothalamus
In humans, a mature ovarian follicle releases an immature egg called the oocyte into the uterine tube approximately every 28 days
Ovaries
It is a flattened, ovoid structure located deep in the pelvic cavity
One section of the ovary is attached to the broad ligament by a peritoneal fold called the mesovarium, and another section to the uterine wall by an ovarian ligament
The ovarian surface is covered by a single layer of cells called the germinal epithelium that overlies the dense irregular connective tissue tunica albuginea
Located below the tunica albuginea is the cortex of the ovary, deep to the cortex is the highly vascularized, connective tissue core of the ovary, the medulla
There is no distinct boundary line between the cortex and medulla, and these regions blend together
Germ cells colonize the gonadal ridges, differentiate into oogenesis, divide by mitosis, and then enter the first phase of meiotic division without completing it
They become arrested in this state of development and are now called the primary oocytes
Primordial follicles - also formed during fetal life, and consist of a primary oocyte surrounded by a single layer of squamous follicular cells
The primordial follicles grow and enlarge to become primary, secondary, and the large mature follicles, which can span the cortex and extend deep into the medulla of the ovary
Ovary may contain a large corpus luteum of an ovulated follicle and a corpus albicans of a degenerate corpus luteum
Ovarian follicles in various stages of development (primordial, primary, secondary, and maturation) may undergo a process of degeneration called atresia, and the atretic, degenerating cells are then phagocytosed by macrophages
Uterine (Fallopian) Tubes
The human uterus - is a pear-shaped organ with a thick muscular wall
Body or corpus - forms the major portion of the uterus
The rounded upper portion of the uterus, located above the entrance of the uterine tubes, is called the fundus
The lower, narrower, and terminal portion of the uterus, located below the body or corpus, is the cervix
The cervix produces and opens into the vagina
The wall of the uterus is composed of three layers:
Outer perimetrium - lined by serosa or adventitia
Myometrium - a thick, smooth muscle layer
Endometrium - the innermost layer- is lined by simple epithelium that descends into a lamina propria to form numerous uterine glands
It is normally subdivided into two functional layers
Luminal stratum functionalis
Basal stratum basalis
The superficial functionalis layer with the iterine glands and blood vessels is sloughed off or shed during menstruation.
The arterial supply to the endometrium plays an important role during the menstrual phase of the menstrual cycle.e
Uterine arteries in the broad ligament give rise to the arcuate arteries
Arcuate vessels give rise to straight and spiral arteries that supply the endometrium
Straight arteries - are short and supply the basalis layer of the endometrium
Spiral arteries - are long and coiled and supply the surface or functionalis layer of endometrium; they are highly sensitive to hormonal changes in the blood
Ovary: Different Stages of Follicular Development (Panoramic View)
The ovary is covered by a single layer of low cuboidal or squamous cells called germinal epithelium → continuous with the mesothelium of the visceral peritoneum → beneath the germinal epithelium is a dense connective tissue layer called tunica albuginea
It has a peripheral cortex and a central medulla, which are found with numerous blood vessels, nerves, and lymphatics
Medulla - is a typical dense irregular connective tissue that is continuous with the mesovarium ligament that suspends the ovary
Larger blood vessels in the medulla distribute smaller vessels to all parts of the ovarian cortex
The mesovarium - covered by the germinal and peritoneal mesothelium
Most of the numerous follicles are the primordial follicles, which are located in the periphery of the cortex and inferior to the tunica albuginea
Primordial follicles - are the smallest and simplest in structure
They are surrounded by a single layer of squamous follicular cells
They contain the immature and small primary oocyte
Primary follicles - cuboidal, columnar, or stratified cuboidal cells that surround the primary oocytes
As the follicles increase in size, a fluid called liquor folliculi (follicular fluid) begins to accumulate between the follicular cells, called the granulosa cells
The fluid areas eventually coalesce to form a fluid-filled cavity, called an antrum
All larger follicles, including primary follicles, secondary follicles, and mature follicles, exhibit a granulosa cell layer, a theca interna, and an outer connective tissue layer, the theca externa
Secondary (antral) follicles - follicles with antral cavities
These are larger and are situated deeper in the cortex
Mature follicle -the largest ovarian follicle
It exhibits the following structures: a large antrum filled with liquor folliculi (follicular fluid); cumulus oophorus, a mound on which the primary oocyte is located; granulosa cells that surround the antrum; the inner layer, theca interna, and the outer layer, theca externa
After ovulation, the large follicle collapses and transforms into a temporary endocrine organ, the corpus luteum
The granulosa cells of the follicle are transformed into light-staining theca lutein cells of the functioning corpus luteum
Corpus albicans - where the corpus luteum regresses, degenerates, and ultimately turns into connective tissue
Most ovarian follicles do not attain maturity; instead, they undergo degeneration (atresia) at all stages of follicular growth and become atretic follicles, which eventually are replaced by connective tissue.
Functional Correlations: Ovaries
Beginning at puberty and during the reproductive years of the individual, the ovaries exhibit structural and functional changes during each menstrual cycle, which lasts an average of 28 days.
These changes involve the growth of different follicles, maturation of follicles, completion of the first meiotic division, ovulation of a secondary oocyte from a mature, dominant follicle, and formation and degeneration of the corpus luteum. The pituitary hormones FSH and LH are primarily responsible for the development, maturation, and ovulation of ovarian follicles and the production of hormones estrogen and progesterone.
The first half of the menstrual cycle lasts about 14 days and involves the growth of ovarian follicles. During follicular growth, the follicular cells possess FSH receptors. At this time, FSH is the principal circulating gonadotrophic hormone. FSH controls the growth and maturation of ovarian follicles, and initially stimulates the theca interna cells around the follicular peripheries to produce androgenic steroid precursors. The androgenic precursors diffuse into the follicles, where the granulosa cells of the follicles convert them into estrogen.
As the follicles develop and mature, the circulating levels of estrogen in the blood rise. Increased levels of estrogen inhibit the release of FSH-releasing factor (hormone) from the hypothalamus and decrease the release of FSH from the pituitary gland. In addition, a hormone called inhibin, produced by granulosa cells in ovarian follicles, further inhibits the release of FSH from the pituitary gland.
At midcycle or shortly before ovulation, estrogen levels reach a peak. This peak causes a surge of LH hormone from the adenohypophysis of the pituitary gland. At this time, theca cells and granulosa cells in the follicles have LH receptors. There is also a concomitant smaller release of FSH hormone. Increased blood levels of both LH and FSH cause the following:
Completion of the first meiotic division just before ovulation and liberation of a secondary oocyte into the uterine tube
Final maturation of a mature ovarian follicle and ovulation (rupture) of a secondary oocyte at about the 14th day of the cycle
Collapse of the ovulated follicle and the luteinization or modification of the granulosa lutein cells and theca lutein cells that surrounded the oocyte
Transformation of the postovulatory mature follicle into the corpus luteum, a temporary endocrine organ
Final maturation or second meiotic division of the secondary oocyte occurs only when it is fertilized by a sperm. The liberated secondary oocyte remains viable in the female reproductive tract for about 24 hours before it begins to degenerate without completing the second meiotic division.
Ovary: Maturing Follicles and Initial Formation of Corpus Luteum
At the superior pole of the ovary, a large follicle is present shortly after ovulation and during the initial stages of corpus luteum formation. The follicular wall of the large mature follicle has collapsed on the former antral cavity → The folded granulosa cells that surround the antral cavity are exhibiting a transformation into the granulosa lutein cells → Surrounding the granulosa lutein cells on their periphery are the darker-staining theca lutein cells which are the former theca interna cells of the mature follicle before ovulation.
Other follicles are in different stages of development
Outer cortex (primary follicles) → larger secondary follicles (with enlarged antral cavities) → in the middle of the ovary are three mature follicles (with large antral cavities) → one of these follicles is visible as the primary oocyte → the surrounding cells of the corona radiata → granulosa cells → peripheral theca interna cells
The ovary also exhibits an atretic follicle in the cortex and numerous interstitial cells. The interstitial cells represent the remnants of theca interna cells that persist as individual cells or small groups of cells throughout the cortex following the follicular atresia.
Maturing Follicles and a Section of Corpus Luteum
This higher-power photomicrograph shows a peripheral fragment of an ovary section that was also collected from the European mink. The photomicrograph shows small and numerous developing primordial follicles close to the periphery in the cortex of the ovary.
Also visible among the developing primordial follicles is a maturing follicle with a large liquid-filled antrum.
Pressed to one side of the follicle is a primary oocyte surrounded by the corona radiata.
Located on the periphery of the antrum are granulosa cells surrounded by theca interna cells.
Also visible in this section of the ovary are granulosa lutein cells and peripheral theca lutein cells of the formed corpus luteum. This ovary also exhibits a group of scattered interstitial cells.
Ovary: Ovarian Cortex and Primary and Primordial Follicles
The ovarian surface is covered by a cuboidal germinal epithelium. Located directly beneath the germinal epithelium is a layer of dense connective tissue called the tunica albuginea.
Numerous primordial follicles are located in the cortex below the tunica albuginea.
Each primordial follicle is surrounded by a single layer of squamous follicular cells.
As the follicles grow larger, the follicular cells of the primordial follicles change to cuboidal or low columnar, and the follicles are now called primary follicles.
The developing oocytes also have a large eccentric nucleus with a conspicuous nucleolus.
In the growing or primary follicles, the follicular cells proliferate by mitosis and form layers of cuboidal cells called the granulosa cells that surround the primary oocytes.
A single layer of the granulosa cells that surround the oocyte forms the corona radiata. Between the corona radiata and the oocyte appears the noncellular glycoprotein layer called the zona pellucida. The stromal cells that surround the follicular cells now differentiate into the theca interna layer that is located adjacent to the granulosa cells.
A thin basement membrane (not shown) separates the granulosa cells from the theca interna cells. Many primordial, developing, or mature follicles exhibit degeneration, die, and are lost through a process called atresia.
Numerous blood vessels, such as capillaries, surround the developing follicles and are found in the connective tissue of the cortex.
Ovary: Primary Oocyte and Wall of a Mature Follicle
During the growth of the follicles, fluid begins to accumulate between the granulosa cells that surround the oocyte, forming a fluid-filled cavity, the antrum. The follicle is called a secondary follicle when the antrum is present.
The cytoplasm and nucleus of a primary oocyte and the wall of a fluid-filled mature follicle. A local thickening of the granulosa cells on one side of the follicle surrounds the primary oocyte → and projects into the antrum of the follicle → granulosa cells form a hillock or a mound called the cumulus oophorus. The single layer of granulosa cells that are located immediately adjacent to the primary oocyte → forms the corona radiata.
Between the corona radiata and the cytoplasm of the primary oocyte is a prominent, acidophilic-staining glycoprotein, the zona pellucida.
The granulosa cells surround the antrum and secrete follicular fluid that fills the antrum cavity. Smaller isolated accumulations of the fluid also occur among the granulosa cells as intercellular follicular fluid.
The basal row of granulosa cells rests on a thin basement membrane that separates the granulosa cells from the cells of the theca interna, an inner layer of vascularized, secretory cells of the follicle. Surrounding the cells of the theca interna is the theca externa layer that blends with the connective tissue of the ovarian cortex.
Ovary: Primordial and Primary Follicles
This photomicrograph shows different types of follicles in the cortex of an ovary. The immature primordial follicles consist of a primary oocyte surrounded by a layer of simple squamous follicular cells.
As the primordial follicles grow to become primary follicles, the layer of simple squamous follicular cells around the oocyte changes to a cuboidal layer. In a larger primary follicle, the follicular cells have proliferated into a stratified layer around the oocyte, called granulosa cells.
A prominent layer of glycoprotein, the zona pellucida, develops between the granulosa cells and the immature oocyte. The cells around the developing follicles are also organized into two distinct cell layers, the inner hormone-secreting theca interna and the outer connective tissue layer theca externa.
The theca interna and theca externa are separated from the granulosa cells by a thin basement membrane. Surrounding the follicles in the cortex are cells and fibers of the connective tissue.
Corpus Luteum (Panoramic view)
At a higher magnification, the corpus luteum is a collapsed and folded mass of glandular epithelium, primarily consisting of theca lutein cells and granulosa lutein cells.
Theca lutein cells extend along the connective tissue septa into the folds of the corpus luteum.
The theca externa cells form a poorly defined capsule around the corpus luteum that also extends inward with the connective tissue septa into folds.
The center of the corpus luteum, or the former follicular cavity, contains remnants of follicular fluid, serum, blood cells, and loose connective tissue with blood vessels from the theca externa that has proliferated and extended into the layers of the glandular epithelium.
The connective tissue also covers the inner surface of the granulosa lutein cells and then spreads throughout the core of the corpus luteum.
Some corpora lutea may contain a postovulatory blood clot in the former follicular cavity. The connective tissue of the cortex that surrounds the corpus luteum contains numerous blood vessels.
Corpus Luteum: Theca Lutein Cells and Granulosa Lutein Cells
The granulosa lutein cells represent the hypertrophied former granulosa cells of the mature follicle and constitute the highly folded mass of the corpus luteum. The granulosa lutein cells are large, have large vesicular nuclei, and stain lightly owing to lipid inclusions.
The theca lutein cells (the former theca interna cells) are located external to the granulosa lutein cells on the periphery of the glandular epithelium. The theca lutein cells are smaller than the granulosa lutein cells, and their cytoplasm stains darker.
Also, the nuclei of theca lutein cells are smaller and darker.
The theca externa with numerous blood vessels, venule and arteriole, and capillaries, invades the granulosa lutein cells and theca lutein cells.
A fine connective tissue septum with fibrocytes penetrates the theca lutein cells. The fibrocytes in the septum between the theca lutein cells can be identified by their elongated and flattened appearance
Functional Correlations: Corpus Luteum
After ovulation of a mature follicle and the liberation of a secondary oocyte into the infundibulum of the uterine tube → the wall of the ruptured follicle collapses and becomes highly folded, → the ovary enters the luteal phase (during this phase, LH secretion induces hypertrophy and transformation of the granulosa cells and theca interna cells of the ovulated follicle into granulosa lutein cells and theca lutein cells, respectively.
These changes transform the ovulated follicle into a temporary endocrine tissue, the corpus luteum. LH continues to stimulate and regulate the cells of the corpus lutein to secrete estrogen and large amounts of progesterone → High levels of estrogen and progesterone further stimulate the development of the uterus and mammary glands in anticipation of implantation of a fertilized egg and pregnancy.
Rising levels of estrogen and progesterone produced by the corpus luteum inhibit further release of FSH and LH, influencing both the neurons in the hypothalamus and gonadotrophs in the adenohypophysis → preventing further ovulation.
If the ovulated secondary oocyte is not fertilized, the corpus luteum continues to secrete its hormones for about 12 days and begins to regress. After its regression, it is called the corpus luteum of menstruation, which eventually becomes a nonfunctional scar tissue called the corpus albicans.
With the decreased functions of the corpus luteum, estrogen and progesterone levels decline, affecting the blood vessels in the endometrium of the uterus and resulting in the shedding of the stratum functionalis of the endometrium, followed by the menstrual flow.
As the corpus luteum ceases function → the inhibitory effects of estrogen and progesterone on the hypothalamus and pituitary gland cells are removed. As a result, FSH is again released from the adenohypophysis → initiating a new ovarian cycle of follicular development and maturation.
If fertilization of the oocyte and implantation of the embryo occur, the corpus luteum increases in size and becomes the corpus luteum of pregnancy →The hormone human chorionic gonadotropin (HCG) secreted by the trophoblast cells of the implanting embryo continues to stimulate the corpus luteum and prevents its regression.
The influence of HCG is similar to that produced by LH from the pituitary gland. As a result, the corpus luteum of pregnancy persists for several months →. As the pregnancy progresses, the function of the corpus luteum is gradually taken over by the placenta, which begins to secrete sufficient amounts of estrogen and progesterone to maintain the pregnancy until parturition.
Uterine Tube: Ampulla With the Mesosalpinx Ligament (Panoramic View, Transverse Section)
The paired, muscular uterine (fallopian) tubes extend from the proximity of the ovaries to the uterus.
On one end, the infundibulum opens into the peritoneal cavity adjacent to the ovary. The other end penetrates the uterine wall to open into the interior of the uterus.
The uterine tubes conduct the ovulated oocyte toward the uterus.
The ampulla is the longest part of the tube and is normally the site of fertilization.
The mucosa of the ampulla exhibits the most extensive mucosal folds.
These folds form an irregular lumen in the uterine tube that produces deep grooves between the folds.
These folds become smaller as the uterine tube nears the uterus.
The mucosa of the uterine tube consists of simple columnar ciliated and nonciliated epithelium that overlies the loose connective tissue lamina propria.
The muscularis consists of two smooth muscle layers, an inner circular layer and an outer longitudinal layer.
The interstitial connective tissue is abundant between the muscle layers, and, as a result, the smooth muscle layers—especially the outer layer—are not distinct.
Numerous venules and arterioles are visible in the interstitial connective tissue.
The serosa of the visceral peritoneum forms the outermost layer on the uterine tube, which is connected to the mesosalpinx ligament of the superior margin of the broad ligament.
Uterine Tube: The Mucosal Folds
A higher magnification of the mucosal folds of the uterine tube shows that the lining epithelium consists of ciliated cells and nonciliated peg (secretory) cells.
The ciliated cells are most numerous in the infundibulum and ampulla of the uterine tube.
The beat of the cilia is directed toward the uterus.
Under the epithelium is seen a prominent basement membrane and the lamina propria with numerous blood vessels.
The lamina propria is a cellular, loose connective tissue with fine collagen and reticular fibers.
During the early proliferative phase of the menstrual cycle and under the influence of estrogen, the ciliated cells undergo hypertrophy, exhibit cilia growth, and become predominant. In addition, there is an increase in the secretory activity of the nonciliated peg cells.
The epithelium of the uterine tube shows cyclic changes, and the proportion of ciliated and nonciliated cells varies with the stages of the menstrual cycle.
Uterine Tube: Lining Epithelium
A higher-magnification photomicrograph illustrates a section of the uterine tube wall with complex mucosal folds that are lined by a simple columnar epithelium.
The luminal epithelium consists of two cell types, the ciliated cells and the nonciliated peg cells with apical bulges that extend above the cilia.
A thin basement membrane separates the luminal epithelium from the underlying vascularized connective tissue that forms the core of the mucosal folds.
A portion of the inner circular smooth muscle layer that surrounds the uterine tube is visible in the periphery on the left side of the illustration.
Functional Correlations
The uterine tubes perform several important reproductive functions. Just before ovulation and rupture of the mature follicle, the fingerlike fimbriae of the infundibulum that are very close to the ovary sweep its surface to capture the released oocyte.
This function is accomplished by gentle peristaltic contractions of smooth muscles in the uterine tube wall and fimbriae.
In addition, the heavily ciliated cells on the fimbriae surface create a current toward the uterus that guides the oocyte into the infundibulum of the uterine tube.
The cilia action and the muscular contractions in the wall of the uterine tube transport the captured oocyte or fertilized egg through the remaining regions of the uterine tube toward the uterus.
The uterine tubes also serve as the site of oocyte fertilization, which normally occurs in the upper region of the ampulla.
The nonciliated or peg cells in the uterine tube are secretory and contribute important nutritive material for the oocyte, the initial development of the fertilized ovum, and the embryo.
The uterine secretions also maintain the viability of sperm in the uterine tubes and allow them to undergo capacitation, a complex biochemical and structural process that activates the sperm and enables them to fertilize the released oocyte.
The epithelium in the uterine tubes exhibits changes that are associated with the ovarian cycle. The height of the uterine tube epithelium is at its maximum during the follicular phase, at which time the ovarian follicles are maturing and circulating levels of estrogen are high.
Uterus: Proliferative (Follicular Phase)
The surface of the endometrium is lined with a simple columnar epithelium overlaying the thick lamina propria.
The lining epithelium extends down into the connective tissue of the lamina propria and forms long, tubular uterine glands.
In the proliferative phase, the uterine glands are usually straight in the superficial portion of the endometrium, but may exhibit branching in the deeper regions near the myometrium.
As a result, numerous uterine glands are seen in cross-section.
The wall of the uterus consists of three layers: the inner endometrium, a middle layer of smooth muscle myometrium, and the outer serous membrane perimetrium (not illustrated).
The endometrium is further subdivided into two zones or layers: a narrow, deep basalis layer adjacent to the myometrium and the functionalis layer, a wider, superficial layer above the basalis layer that extends to the lumen of the uterus.
During the menstrual cycle, the endometrium exhibits morphologic changes that are directly correlated with ovarian function. The cyclic changes in a nonpregnant uterus are divided into three distinct phases: the proliferative (follicular) phase, the secretory (luteal) phase, and the menstrual phase.
In the proliferative phase of the cycle and under the influence of ovarian estrogen, the stratum functionalis increases in thickness, and the uterine glands elongate and follow a straight course to the surface.
Also, the coiled (spiral) arteries (in cross section) are primarily seen in the deeper regions of the endometrium.
The lamina propria in the upper regions of the endometrium is cellular and resembles mesenchymal tissue.
The connective tissue in the basilis layer is more compact and appears darker in this illustration.
The endometrium continues to develop during the proliferative phase as a result of the increasing levels of estrogen secreted by the developing ovarian follicles.
The endometrium is situated above the myometrium, which consists of compact bundles of smooth muscle separated by thin strands of interstitial connective tissue with numerous blood vessels.
As a result, the muscle bundles are seen in cross, oblique, and longitudinal sections.
Uterus: Secretory (Luteal) Phase
The secretory (luteal) phase of the menstrual cycle is initiated after ovulation of the mature follicle.
Additional changes in the endometrium are caused by the influence of estrogen and progesterone, which are secreted by the functioning corpus luteum.
As a result, the functionalis layer and basalis layer of the endometrium become thicker owing to increased glandular secretion and edema in the lamina propria.
The epithelium of the uterine glands undergoes hypertrophy (enlargement) as a result of increased accumulation of the secretory product.
The uterine glands also become highly coiled (tortuous), and their lumina become dilated with nutritive secretory material rich in carbohydrates.
The coiled arteries continue to extend into the upper portion of the endometrium (functionalis layer) and become prominent because of their thicker walls.
The alterations in the surface columnar epithelium, uterine glands, and lamina propria characterize the functionalis layer of the endometrium during the secretory or luteal phase of the menstrual cycle.
The basalis layer exhibits minimal changes.
Below the basalis layer is the myometrium with smooth muscle bundles, sectioned in both longitudinal and transverse planes, and blood vessels
Uterine Wall (Endometrium): Secretory (Luteal) Phase
A low-power photomicrograph illustrates a section of the endometrium during the secretory (luteal) phase of the menstrual cycle.
The thick and lighter area of the endometrium is the stratum functionalis.
The darker and deeper endometrium is the stratum basalis.
During the secretory phase, the uterine glands are coiled (tortuous) and secrete glycogen-rich nutrients into their lumina.
Surrounding the uterine glands is the highly cellular connective tissue.
The light, empty spaces in the connective tissue layer are caused by increased edema in the endometrium.
Below the stratum basalis is the smooth muscle layer, myometrium of the uterine wall
Uterus: Menstrual Phase
If fertilization of the ovum and implantation of the embryo do not occur, the uterus enters the menstrual phase, and many of the preparatory changes made for implantation in the endometrium are lost.
During the menstrual phase, the endometrium in the functionalis layer degenerates and is sloughed off.
The shed endometrium contains fragments of disintegrated stroma, blood clots, and uterine glands.
Some of the intact uterine glands are filled with blood.
In the deeper layers of the endometrium, the basalis layer and the bases of the uterine glands remain intact during the shedding of the functionalis layer and the menstrual flow.
The endometrial stroma of most of the functionalis layer contains aggregations of erythrocytes that have been extruded from the torn and disintegrating blood vessels.
In addition, the endometrial stroma exhibits infiltration of lymphocytes and neutrophils.
The basalis layer of the endometrium remains unaffected during this phase.
The distal (superficial) portions of the coiled arteries become necrotic, whereas the deeper parts of these vessels remain intact.
Functional Correlations: Uterus
During pregnancy, the uterus provides the site for implantation of the embryo, formation of the placenta, and a suitable environment for the development of the embryo and fetus.
The endometrium also exhibits cyclical changes in its structure and function in response to the ovarian hormones estrogen and progesterone.
The uterine changes are associated with impending implantation and nourishment of the developing organism.
If fertilization of the oocyte and implantation of the embryo do not occur, blood vessels in the endometrium deteriorate and rupture, and the functionalis layer of endometrium is shed as part of the menstrual flow or discharge. With each menstrual cycle during the reproductive period of the individual, the endometrium passes through three phases, with each phase gradually passing into the next.
The proliferative (preovulatory, follicular phase) is characterized by rapid growth and development of the endometrium.
The resurfacing and growth of the endometrium during the proliferative phase closely coincide with the rapid growth of ovarian follicles and their increased production of estrogen.
This phase starts at the end of the menstrual phase, or about day 5, and continues to about day 14 of the cycle.
Increased mitotic activity of the lamina propria and remnants of the uterine glands in the basalis layer of the endometrium produce new cells that begin to cover the raw surface of the uterine mucosa that was denuded or shed during menstruation.
The resurfacing of the mucosa produces a new functional layer of the endometrium.
As the functional layer thickens, the uterine glands proliferate, lengthen, and become closely packed.
The spiral arteries begin to grow toward the endometrial surface and begin to show light coiling.
The secretory (postovulatory, luteal phase) begins shortly after ovulation, on about day 15, and continues to about day 28 of the cycle.
This phase is dependent on the functional corpus luteum that was formed after ovulation and the secretion of progesterone and estrogen by the lutein cells (granulosa lutein and theca lutein cells).
During the postovulatory phase, the endometrium thickens and accumulates fluid, becoming edematous.
In addition, the uterine glands undergo hypertrophy and become tortuous, and their lumina become filled with secretions rich in nutrients, especially glycoproteins and glycogen.
The spiral arteries in the endometrium also lengthen, become more coiled, and extend almost to the surface of the endometrium.
The menstrual (menses) phase of the cycle begins when the ovulated oocyte is not fertilized and no implantation occurs in the uterus.
Reduced levels of circulating progesterone (and estrogen), as a result of the regressing corpus luteum, initiate this phase.
Decreased levels of these hormones induce intermittent constrictions of the spiral arteries and interruption of blood flow to the functionalis layer of the endometrium, while the blood flow to the basalis layer remains uninterrupted.
These constrictions deprive the functionalis layer of oxygenated blood and produce transitory ischemia, causing necrosis (death) of cells in the walls of blood vessels and degeneration of the functionalis layer in the endometrium.
After extended periods of vascular constriction, the spiral arteries dilate, resulting in the rupture of their necrotic walls and hemorrhage (bleeding) into the stroma.
The necrotic functionalis layer then detaches from the rest of the endometrium. Blood, uterine fluid, stromal cells, secretory material, and epithelial cells from the functionalis layer mix to form the menstrual flow.
The shedding of the functionalis layer of the endometrium continues until only the raw surface of the basalis layer is left.
The remnants of uterine glands in the basalis layer serve as the source of cells for regenerating the next functionalis layer.
Rapid proliferation of cells in the glands of the basalis layer, under the influence of rising estrogen levels during the proliferative phase, resurfaces and restores the lost endometrial layer and starts the next phase of the menstrual cycle.
Summary of the Overview of the Female Reproductive System
Overview of the Female Reproductive System
Consists of paired ovaries, uterine tubes, and a single uterus
Uterus separated from vagina by cervix
Organs exhibit cyclical monthly changes in the form of the menstrual cycle
The start of the first cycle is the menarche, and the end of the cycle is the menopause
Cycles are controlled by hormones FSH and LH, and ovarian estrogen and progesterone
An immature oocyte is released about every 28 days into the uterine tube
Ovaries
Germinal epithelium overlies the connective tissue tunica albuginea
Consists of an outer cortex and inner medulla, without distinct boundaries
During embryonic development, oogonia divide by mitosis in the gonadal ridges
Oogonia enter the first meiotic division and remain as primary oocytes in primordial follicles
At puberty, primordial follicles grow to become primary, secondary, and mature follicles
Ovarian follicles can become atretic at any stage of development
Follicular Developments in Ovary
Primordial follicles with a primary oocyte are surrounded by squamous follicular cells.
Primary follicles exhibit simple cuboidal or stratified granulosa cell layers.
Secondary follicles exhibit liquid accumulations between granulosa cells or an antrum.
Largest follicles are mature, span the cortex, and extend into the medulla
In maturing follicles, oocytes are located on the mound of the cumulus oophorus
Theca interna and theca externa are visible in larger, developing follicles
Primary oocytes are surrounded by zona pellucida and corona radiata cells in follicles
FSH and LH are responsible for the development, maturation, and ovulation of follicles
During the first half of the menstrual cycle and during follicular growth, FSH is the principal hormone
FSH controls the growth of follicles and stimulates estrogen production from follicles
At midcycle, estrogen levels peak and cause the release of LH
FSH and LH cause the final maturation and ovulation of the dominant, mature follicle
At ovulation, first meiotic division is completed, and the secondary oocyte is released
Ovulation site on the mature follicle is the thinned cell area called the stigma
Ovulated follicle collapses and becomes a temporary corpus luteum
Completion of the second meiotic division occurs only when the oocyte is fertilized by sperm
An oocyte is viable for about 24 hours before it degenerates if not fertilized
Interstitial cells in the ovary are remnants of theca interna cells after follicular atresia
Corpus Luteum
Forms after ovulation and liberation of secondary oocyte
LH induces hypertrophy and luteinization of granulosa and theca interna cells
LH causes liberation of estrogen and increased amounts of progesterone
Without fertilization, is active for about 12 days before regression
Regression eventually leads to connective scar tissue corpus albicans
After regression, inhibitory effects of estrogen and progesterone are removed
FSH and LH are again released to start new ovarian cycle
If fertilization occurs, corpus luteum of pregnancy forms
Human chorionic gonadotropin produced by trophoblasts stimulates corpus luteum
Persists during pregnancy until placenta produces estrogen and progesterone
Uterine Tubes
Extend from ovaries into the uterus and exhibit four continuousregions
Infundibulum with fimbriae of the uterine tube located adjacent to the ovary
Mucosa consists of extensive folds and forms irregular lumen
Epithelium simple columnar with ciliated and nonciliated secretory (peg) cells
Ciliated cells create a current toward uterus and become predominant in proliferative phase
Secretory cells provide nutrition for oocyte, fertilized ovum, and developing embryo
Uterine tube secretions maintain sperm and enhance capacitation of sperm
Smooth muscles provide peristaltic contractions to help capture ovulated oocyte
Epithelium exhibits changes associated with ovarian cycle
Uterus
Consists of body, fundus, and cervix
Wall consists of outer perimetrium, middle myometrium, and inner endometrium
Endometrium divided into stratum functionalis and stratum basalis
During monthly menstrual cycles, stratum functionalis is shed with menstrual flow
Endometrium morphology responds to estrogen and progesterone and ovarian functions
Proliferative phase starts at the end of menstrual phase after estrogen release
Ovarian estrogen induces endometrial growth and formation of new stratum functionalis
Secretory phase starts after ovulation and corpus luteum formation
Estrogen and increased progesterone levels induce uterine gland secretion of nutrients
Spiral arteries extend and reach surface of endometrium
Menstrual phase starts when ovulated oocyte is not fertilized and no implantation occurs
Spiral arteries highly sensitive to declining hormone levels and constrict intermittently
Ischemia destroys walls of blood vessels and stratum functionalis
Dilation of spiral arteries ruptures walls, detaches functionalis, and causes menstruation
Stratum basalis remains intact and is not shed during menstruation
Stratum basalis serves as the source of cells for regenerating new stratum functionalis
Section 2: Cervix, Vagina, Placenta, and Mammary Glands
Cervix and Vagina
The cervix is located in the lower part of the uterus that projects into the vaginal canal as the portio vaginalis.
A narrow cervical canal passes through the cervix. The opening of the cervical canal that directly communicates with the uterus is the internal os and, with the vagina, the external os.
Unlike the functionalis layer of the uterine endometrium, the cervical mucosa undergoes only minimal changes during the menstrual cycle and is not shed during menstruation.
The cervix contains numerous branched cervical glands that exhibit altered secretory activities during the different phases of the menstrual cycle.
The amount and type of mucus secreted by the cervical glands change during the menstrual cycle as a result of different levels of ovarian hormones.
The vagina is a fibromuscular structure that extends from the cervix to the vestibule of the external genitalia.
Its wall has numerous folds and consists of an inner mucosa, a middle muscular layer, and an outer connective tissue adventitia.
The vagina does not have any glands in its wall and its lumen is lined by stratified squamous epithelium.
Mucus produced by cells in the cervical glands lubricates the vaginal lumen.
Loose fibroelastic connective tissue and a rich vasculature constitute the lamina propria that overlies the smooth muscle layers of the organ.
Like the cervical epithelium, the vaginal lining is not shed during the menstrual flow.
Placenta
The placenta is a temporary organ that is formed when the developing embryo, now called a blastocyst, attaches to and implants in the endometrium of the uterus.
The placenta consists of a fetal portion, formed by the chorionic plate and its branching chorionic villi, and a maternal portion, formed by the decidua basalis of the endometrium.
Fetal and maternal blood come into close proximity in the villi of the placenta.
Exchange of nutrients, electrolytes, hormones, antibodies, gaseous products, and waste metabolites takes place as the blood passes over the villi.
Fetal blood enters the placenta through a pair of umbilical arteries, passes into the villi, and returns through a single umbilical vein
Mammary Glands
The adult mammary gland is a compound tubuloalveolar gland that consists of about 20 lobes.
All lobes are connected to lactiferous ducts that open at the nipple.
The lobes are separated by connective tissue partitions and adipose tissue.
The resting or inactive mammary glands are small, consist primarily of ducts, and do not exhibit any developed or secretory alveoli.
Inactive mammary glands also exhibit slight cyclic alterations during the course of the menstrual cycle.
Under estrogenic stimulation, the secretory cells increase in height, lumina appear in the ducts, and a small amount of secretory material is accumulated
Cervix, Cervical Canal, and Vaginal Fornix (Longitudinal Section)
The cervix is the lower part of the uterus.
The cervical canal is lined with tall,mucus-secreting columnar epithelium that is different from the uterine epithelium, with which it is continuous.
The cervical epithelium also lines the highly branched and tubular cervical glands that extend at an oblique angle to the cervical canal into the lamina propria
Some of the cervical glands may become occluded and develop into small glandular cysts.
The connective tissue in the lamina propria of the cervix is more fibrous than in the uterus.
Blood vessels, nerves, and occasional lymphatic nodules may be seen.
The lower end of the cervix, the os cervix, bulges into the lumen of the vaginal canal
The columnar epithelium of the cervical canal abruptly changes to nonkeratinized
stratified squamous epithelium to line the vaginal portion of the cervix called the portio vaginalis and the external surface of the vaginal fornix.
At the base of the fornix, the epithelium of the vaginal cervix reflects back to become the vaginal epithelium of the vaginal wall
The smooth muscles of the muscularis extend into the cervix but are not as compact as the muscles in the body of the uterus.
Functional Correlations: Cervix
The cervical mucosa does not undergo extensive changes during the menstrual cycle.
However, the cervical glands exhibit functional changes that are related to sperm transport through the cervical canal.
During the proliferative phase of the menstrual cycle, the secretion from the cervical glands is thin and watery.
This type of secretion allows for easier passage of sperm through the cervix and into the uterus.
During the secretory (luteal) phase of the menstrual cycle and increased progesterone secretions, as well as during pregnancy, the cervical gland secretions change and become highly viscous, forming a mucus plug in the cervical canal.
The mucus plug is a protective measure that hinders the passage of sperm and microorganisms from the vagina into the body of the uterus.
Thus, the cervical glands perform an important function in assisting fertilization of the oocyte and protection of the developing individual.
Vagina (Longitudinal Section)
The vaginal mucosa is irregular and shows mucosal folds.
The surface epithelium of the vaginal canal is noncornified stratified squamous.
The underlying connective tissue papillae are prominent and indent the epithelium.
The lamina propria contains dense, irregular connective tissue with elastic fibers that extend into the muscularis layer as interstitial fibers
Diffuse lymphatic tissue, lymphatic nodules, and small blood vessels are in the lamina propria.
The muscularis of the vaginal wall consists predominantly of longitudinal bundles and oblique bundles of smoothmuscle.
The transverse bundles of the smooth muscle are less numerous but more frequently found in the inner layers.
The interstitial connective tissue is rich in elastic fibers.
Blood vessels and nerve bundles are abundant in the adventitia
Glycogen in Human Vaginal Epithelium
Glycogen is a prominent component of the vaginal epithelium, except in the deepest layers, where it is minimal or absent.
During the follicular phase of the menstrual cycle, glycogen accumulates in the vaginal epithelium, reaching its maximum level before ovulation.
Glycogen can be demonstrated by iodine vapor or iodine solution in mineral oil (Mancini method); glycogen stains a reddish purple.
The amount of glycogen in the vaginal epithelium is illustrated during the interfollicular
phase (a).
During the follicular phase (b), glycogen content increases in the intermediate and superficial cell layers.
The tissue sample in illustration (c) is from the same specimen as in (b), but was fixed by the Altmann-Gersh method (freezing and drying in a vacuum).
This method produces less tissue shrinkage and illustrates more glycogen and its diffuse distribution in the vaginal epithelium during the follicular phase (c).
Functional Correlations: Vagina
The wall of the vagina consists of mucosa, a smooth muscle layer, and an adventitia.
There are no glands in the vaginal mucosa.
The surface of the vaginal canal is kept moist and lubricated by secretions produced by cervical glands.
The vaginal epithelium exhibits minimal changes during each menstrual cycle.
During the proliferative (follicular) phase of the menstrual cycle and owing to increased estrogen stimulation, the vaginal epithelium increases in thickness.
In addition, estrogen stimulates the vaginal cells to synthesize and accumulate increased amounts of glycogen as these cells migrate toward the vaginal lumen, into which they are shed or desquamated.
Bacterial flora in the vagina metabolizes glycogen into lactic acid.
Increased acidity in the vaginal canal protects the organ against microorganisms or pathogenic invasion.
Microscopic examination of cells collected (scraped) from the vaginal and cervical
mucosae, called a Pap smear, provides highly valuable diagnostic information of clinical importance.
Cervicovaginal Pap smears are routinely examined for early detection of pathologic
changes in the epithelium of these organs that may lead to cervical cancer.
Vagina Exfoliate Cytology (Vaginal Smear) During Different Reprodcutive Phases
Vaginal exfoliate cytology (vaginal smear) is closely correlated with the ovarian cycle.
The presence of certain cell types in the smear permits the recognition of the follicular activity during normal menstrual phases or after hormonal therapy.
Also, exfoliate cytology together with cells from the endocervix provides a very important source of information for early detection of cervical or vaginal cancers.
A combination of hematoxylin, orange G, and eosin azure facilitates the recognition of different cell types.
In most phases, the surface squamous cells show small, dark-staining pyknotic nuclei and increased amount of cytoplasm.
Figure a illustrates vaginal cells collected during the postmenstrual phase (fifth day of the menstrual cycle).
The intermediate cells (1) from the intermediate cell layers (precornified superficial vaginal cells) predominate.
In addition, a few superficial acidophilic (2) cells and leukocytes are present.
Figure b represents a vaginal smear collected during the ovulatory phase (14th day) of the menstrual cycle.
There is a scarcity of intermediate cells (8) and an absence of leukocytes.
The large superficial acidophilic cells (9) characterize this phase.
This smear characterizes the results of the high estrogenic stimulation normally observed before ovulation.
The superficial acidophilic cells (8) mature with increased estrogen levels and become acidophilic.
A similar type of smear is seen when a menopausal woman is treated with high doses of estrogen.
Figure c represents a vaginal smear collected during the luteal (secretory) phase and represents the effects of increased levels of progesterone.
The large intermediate cells (3) with folded borders aggregate into clumps and characterize the smear.
Superficial acidophilic cells (4) and leukocytes are scarce
Figure d represents a vaginal smear taken during the premenstrual phase.
This stage is characterized by a predominance of grouped intermediate cells with folded borders, an increase in the number of the neutrophils, a scarcity of the superficial acidophilic cells (12), and an abundance of mucus
Figure e illustrates a vaginal smear taken during early pregnancy.
The cells exhibit dense groups or conglomerations of predominantly intermediate cells with folded borders.
Superficial acidophilic cells and neutrophils are scarce
The vaginal smear collected during menopause in Figure f is different from all other phases.
The intermediate cells are scarce,whereas the predominant cells are the oval basal cells.
Also, neutrophils are in abundance. Menopausal smears are variable and depend on the stage of the menopause and the estrogen levels
Functional Correlations: Cellular Characteristics of Vaginal Cytology (Smear)
The superficial acidophilic cells of the vaginal epithelium appear flat and irregular in outline, measuring about 35 to 65 μm in diameter, exhibit small pyknotic nuclei, and contain cytoplasm that is stained light red (acidophilic) or orange.
The intermediate cells are flat like the superficial cell, but are somewhat smaller, measuring 20 to 40 μm in diameter, and show a basophilic blue-green cytoplasm. The nuclei are somewhat larger than those of the superficial cells, and are often vesicular.
The intermediate cells are also elongated with folded borders and elongated, eccentric nuclei.
The larger basal cells are from the basal layers of the vaginal epithelium.
All basal cells are oval, measure from 12 to 15 μm in diameter, and exhibit large nuclei with prominent chromatin.
Most of these cells exhibit basophilic staining.
Vagina: Surface Epithelium
This higher-magnification photomicrograph illustrates the vaginal epithelium and the underlying connective tissue.
The surface epithelium is stratified squamous nonkeratinized.
Most of the superficial cells in vaginal epithelium appear empty owing to increased accumulation of glycogen in their cytoplasm.
During histologic preparation of the organ, the glycogen was extracted by chemicals.
The lamina propria contains dense, irregular connective tissue.
The lamina propria lacks glands but contains numerous blood vessels and lymphocytes
Human Placenta (Panoramic View)
The upper region of the figure illustrates the fetal portion of the placenta, which includes the chorionic plate (1) and the chorionic villi (2, 10, 12, 14).
The maternal part of the placenta is the decidua basalis of the endometrium that lies directly beneath the fetal placenta.
The amniotic surface is lined by simple squamous epithelium, below which is the connective tissue of the chorion.
Inferior to the connective tissue layer are the trophoblast cells of the chorion.
The trophoblasts and the underlying connective tissue form the chorionic plate.
The anchoring chorionic villi arise from the chorionic plate, extend to the uterine wall, and attach to the decidua basalis.
Numerous floating villi (chorion frondosum), sectioned in various planes, extend in all directions from the anchoring villi.
These villi “float’’ in the intervillous space, which is bathed in maternal blood.
The maternal portion of the placenta, the decidua basalis, contains anchoring villi, large decidual cells, and a typical connective tissue stroma.
The decidua basalis also contains the basal portions of the uterine glands.
The maternal blood vessels in the decidua basalis are recognized by their size or by the presence of blood cells in their lumina.
A maternal blood vessel can be seen opening directly into the intervillous space.
A portion of the smooth muscle myometrium (7) of the uterine wall is visible in the left corner of the illustration.
Chorionic Villi: Placenta During Early Pregnancy
The chorionic villi from a placenta during early pregnancy are illustrated at a higher magnification.
The trophoblast cells of the embryo give rise to the embryonic portion of the placenta.
The chorionic villi arise from the chorionic plate and become surrounded by the trophoblast epithelium that consists of an outer layer of the darker-staining syncytiotrophoblasts and an inner layer of lighter-staining cytotrophoblasts.
The core of each chorionic villus contains mesenchyme or embryonic connective tissue and contains two cell types, the fusiform mesenchyme cells and the darker-staining macrophage (Hofbauer cell).
The fetal blood vessels, branches of the umbilical arteries and veins, are located in the core of the chorionic villi and contain fetal nucleated erythroblasts, although nonnucleated cells can also be seen.
The intervillous space is bathed by maternal blood cells and nonnucleated erythrocytes.
Chronic Villi: Placenta at Term
The chorionic villi are illustrated from a placenta at term.
In contrast to the chorionic villi in the placenta during pregnancy, the chorionic epithelium in the placenta at term is reduced to only a thin layer of syncytiotrophoblasts
The connective tissue in the villi is differentiated with more fibers and fibroblasts, and contains large, round macrophages (Hofbauer cells).
The villi also contain mature blood cells in the fetal blood vessels that have increased in complexity during pregnancy.
The intervillous space is surrounded by maternal blood cells.
Functional Correlations: Placenta
The placenta is an organ that performs an important function in regulating the exchange of different substances between the maternal and fetal circulation during pregnancy.
One side of the placenta is attached to the uterine wall, and on the other side it is attached to the fetus via the umbilical cord.
Maternal blood enters the placenta through blood vessels located in the endometrium and is directed to the intervillous spaces, where it bathes the surface of the villi, which contain the fetal blood.
Here, metabolic waste products, carbon dioxide, hormones, and water are passed from the fetal circulation to the maternal circulation.
Oxygen, nutrients, vitamins, electrolytes, hormones, immunoglobulins (antibodies), metabolites, and other substances pass in the opposite direction.
Maternal blood leaves the intervillous spaces through the endometrial veins.
The placenta also serves as a temporary—yet major—endocrine organ that produces numerous essential hormones for the maintenance of pregnancy.
Placental cells (syncytial trophoblasts) secrete the hormone chorionic gonadotropin shortly after implantation of the fertilized ovum.
In humans, chorionic gonadotropin appears in urine within 10 days of pregnancy, and its presence can be used to determine pregnancy with commercial kits.
Chorionic gonadotropin hormone is similar to luteinizing hormone (LH) in structure and function, and it maintains the corpus luteum in the maternal ovary during the early stages of pregnancy.
Chorionic gonadotropin also stimulates the corpus luteum to produce estrogen and progesterone, the two hormones that are essential for maintaining pregnancy.
The placenta also secretes chorionic somatomammotropin, a glycoprotein hormone that exhibits both lactogenic and growth-promoting functions.
As pregnancy proceeds, the placenta gradually takes over production of estrogen and progesterone from the corpus luteum and produces sufficient amounts of progesterone to maintain the pregnancy until birth.
The placenta also produces relaxin, a hormone that softens the fibrocartilage in the pubic symphysis to widen the pelvic canal for impending birth.
In some mammals, the placenta also secretes placental lactogen, a hormone that promotes growth and development of the maternal mammary glands.
Inactive Mammary Gland
The inactive mammary gland is characterized by an abundance of connective tissue and by a scarcity of the glandular elements.
Some cyclic changes in the mammary gland may be seen during the menstrual cycles.
A glandular lobule consists of small tubules or intralobular ducts lined with a cuboidal or a low columnar epithelium.
At the base of the epithelium are the contractile myoepithelial cells.
The larger interlobular ducts surround the lobules and the intralobular ducts .
The intralobular ducts are surrounded by loose intralobular connective tissue that contains fibroblasts, lymphocytes, plasma cells, and eosinophils.
Surrounding the lobules is a dense interlobular connective tissue containing blood vessels, venule and arteriole.
The mammary gland consists of 15 to 25 lobes, each of which is an individual compound tubuloalveolar type of gland.
Each lobe is separated by dense interlobar connective tissue.
A lactiferous duct independently emerges from each lobe at the surface of the nipple.
Mammary Gland During Proliferation and Early Pregnancy
In preparation for milk secretion (lactation), the mammary gland undergoes extensive structural changes.
During the first half of the pregnancy, the intralobular ducts undergo rapid proliferation and form terminal buds that differentiate into alveoli.
At this stage, most of the alveoli are empty and it is difficult to distinguish between the small intralobular excretory ducts and the alveoli.
The intralobular excretory ducts appear more regular with a more distinct epithelial lining.
The intralobular excretory ducts and the alveoli are lined by two layers of cells, the luminal epithelium and a basal layer of flattened myoepithelial cells.
A loose intralobular connective tissue surrounds the alveoli and the ducts.
A denser connective tissue with adipose cells surrounds the individual lobules and forms interlobular connective tissue septa.
The interlobular excretory ducts, lined with taller columnar cells, course in the interlobular connective tissue septa to join the larger lactiferous duct that is usually lined with low pseudostratified columnar epithelium.
Each lactiferous duct collects the secretory product from the lobe and transports it to the nipple.
Mammary Gland During Late Pregnancy
A small section of a mammary gland with lobules, connective tissue, and excretory ducts is illustrated at lower (left) and higher (right) magnification.
During pregnancy, the glandular epithelium is prepared for lactation.
The alveolar cells become secretory, and the alveoli and the ducts enlarge.
Some of the alveoli contain a secretory product.
However, the secretion of milk by the mammary gland does not begin until after parturition (birth).
Because the intralobular excretory ducts of the mammary gland also contain secretory material, the distinction between alveoli and ducts is difficult.
As pregnancy progresses, the amount of intralobular connective tissue decreases, while the amount of interlobular connective tissue increases because of the enlargement of the glandular tissue.
Surrounding the alveoli are flattened myoepithelial cells, which are more visible in the higher magnification on the right.
Located in the interlobular connective tissue are the interlobular excretory ducts, lactiferous ducts with secretory product in their lumina, various types of blood vessels, and adipose cells.
Mammary Gland During Lactation
The lactating mammary gland contains a large number of distended alveoli filled with secretions and vacuoles.
The alveoli show irregular branching patterns.
Because of the increased size of the glandular epithelium (alveoli), the interlobular connective tissue septa is reduced.
During lactation, the histology of individual alveoli varies.
Not all of the alveoli exhibit secretory activity.
The active alveoli are lined with low epithelium and filled with milk that appears as eosinophilic (pink) material with large vacuoles of dissolved fat droplets.
Some alveoli accumulate secretory product in their cytoplasm, and their apices appear vacuolated because of the removal of fat during tissue preparation.
Other alveoli appear inactive with empty lumina lined by a taller epithelium.
In the mammary gland, the myoepithelial cells (not illustrated) are present between the alveolar cells and the basal lamina.
The contraction of myoepithelial cells expels milk from the alveoli into the excretory ducts.
The interlobular excretory ducts are embedded in the connective tissue septa that contain adipose cells.
Lactating Mammary Gland
The lactating mammary gland contains alveoli with the secretory product milk and separated by thin connective tissue septa.
Some of the alveoli are single, whereas others are branching alveoli.
All of the alveoli eventually drain into larger excretory ducts that eventually deliver the milk to the lactiferous ducts in the nipple.
The mammary glands contain large amounts of adipose tissue during lactation.
Functional Correlations: Mammary Glands
Before puberty, the mammary glands are undeveloped and consist primarily of branched lactiferous ducts that open at the nipple. In males, the mammary glands remain undeveloped.
In females, mammary glands enlarge during puberty because of stimulation by estrogen.
As a result, adipose tissue and connective tissue accumulate and grow, and branching of the lactiferous ducts in the mammary glands increases.
During pregnancy, the mammary glands undergo increased growth owing to the continuous and prolonged stimulatory actions of estrogen and progesterone.
These hormones are initially produced by the corpus luteum of the ovary and later by cells in the placenta.
In addition, further growth of the mammary glands depends on the pituitary hormone prolactin, placental lactogen, and adrenal corticoids.
These hormones stimulate the intralobular ducts of the mammary glands to rapidly proliferate, branch, and form numerous alveoli.
The alveoli then undergo hypertrophy and become active sites of milk production during the lactation period.
All alveoli become surrounded by contractile myoepithelial cells.
At the end of pregnancy, the alveoli initially produce fluid called colostrum that is rich in proteins, vitamins, minerals, and antibodies.
Unlike milk, however, colostrum contains little lipid.
Milk is not produced until a few days after parturition (birth).
The hormones estrogen and progesterone from the corpus luteum and placenta suppress milk production.
After parturition and elimination of the placenta, the hormones that inhibited milk secretion are eliminated and the mammary glands begin active secretion of milk.
As the pituitary hormone prolactin activates milk secretion, the production of colostrum ceases.
During nursing of the newborn, tactile stimulation of the nipple by the suckling infant promotes further release of prolactin and prolonged milk production.
In addition, tactile stimulation of the nipple by the infant initiates the milk ejection reflex that causes the release of the hormone oxytocin from the neurohypophysis of the pituitary gland.
Oxytocin causes the contraction of myoepithelial cells around the secretory alveoli and excretory ducts in the mammary glands, resulting in milk ejection from the mammary glands toward the nipple.
Decreased nursing and suckling by the infant soon results in the cessation of milk production and eventual regression of the mammary glands to an inactive state