reproductive

TOPIC XVII: REPRODUCTIVE SYSTEMS

A) Structures: Overview

Gonads (Testes/Ovaries)- Produce gametes (sperm and oocytes) and sex hormones (e.g., testosterone, estrogen, progesterone).
Ducts- Transport and store gametes from the gonads to the exterior or to other reproductive organs.
Accessory Glands- Produce secretions that support gametes and facilitate their transport.
Refer to Figures 27.4 & 27.14 for related illustrations.

B) Testicular Reproductive System

Gonads:
- Testes are located within the scrotum (a skin and connective tissue structure) outside the abdominopelvic cavity to maintain a temperature about $3$ °C lower than body temperature, which is optimal for spermatogenesis.
- Surrounded by two tunics:
1. Tunica Vaginalis:
  - Outer layer (superficial).
  - A serous membrane derived from peritoneum, consisting of a parietal and visceral layer, that reduces friction between the moving testis and the scrotal wall.
2. Tunica Albuginea:
  - Inner fibrous connective tissue capsule.
  - Extends inward to divide testis into approximately $250-300$ wedge-shaped lobules, each containing $1-4$ seminiferous tubules.
- Each lobule contains:
- Seminiferous Tubules:
  - The actual site of sperm production (spermatogenesis), spanning about $80$ cm when uncoiled.
  - Unite to form rete testis at the posterior side of each testis, a network of tubules that collect sperm.
- Spermatogenic Cells:
  - Germ cells in various stages of development that will become sperm, lining the tubules from the basement membrane to the lumen, undergoing mitosis and meiosis.
- Sustentocytes (Nurse Cells):
  - Large, columnar cells that surround, nourish, and protect developing gametes, extending from the basement membrane to the lumen.
  - Connected by tight junctions, forming the blood-testis barrier (BTB) which protects developing sperm from the immune system (as sperm contain different antigens than somatic cells), toxins, drugs, and fluctuating interstitial fluid composition.
  - Produce testicular fluid for sperm transport in the lumen, rich in androgens and other growth factors.
- Interstitial Endocrine (Leydig) Cells:
  - Located in connective tissue (interstitial space) between seminiferous tubules.
  - Secrete testosterone, primarily starting at puberty, which is essential for initiating and maintaining spermatogenesis, as well as developing and maintaining male secondary sexual characteristics.

Ducts:

From seminiferous tubules, sperm enter rete testis and then into duct system:
1. Epididymis:
- A comma-shaped organ located at the posterior border of testis, approximately $6$ m long when uncoiled.
- Stores sperm (for up to several months) and allows sperm to mature (develop the ability to swim and fertilize an oocyte, a process that takes about $20$ days).
1. Vas (Ductus) Deferens:
- Transports sperm from the epididymis during ejaculation via powerful peristaltic contractions of its thick smooth muscle walls.
- Part of the spermatic cord which carries vas deferens, testicular arteries, pampiniform plexus (venous network), nerves, and lymphatic vessels through the inguinal canal of the body wall.
- Loops over the posterior wall of the bladder, dilating to form the ampulla before joining the duct from the seminal vesicle.
1. Ejaculatory Duct:
- Short duct (about $2$ cm long) formed by the union of the vas deferens (ampulla) and duct from the seminal vesicle.
- Passes through the prostate gland and empties its contents (sperm and seminal fluid) into the prostatic urethra.
1. Urethra:
- Transports urine and semen out of the body (though never simultaneously, as reflexive muscular contractions close off the bladder during ejaculation).
- Divided into three regions:
  - Prostatic Urethra: About $2.5$ cm long, through the prostate gland, receives ejaculatory ducts.
  - Intermediate (Membranous) Urethra: About $1$ cm long, through the urogenital diaphragm.
  - Spongy (Penile) Urethra: The longest section (about $15$ cm), through the corpus spongiosum of the penis, opening at the glans penis (external urethral orifice).

Penis:

Urinary and Copulatory Organ: Designed to deliver sperm to the female reproductive tract during sexual intercourse.
Parts include:
1. Root: The attached portion, including the crura and bulb of the penis.
2. Body (Shaft): The free, movable part, which contains most of the erectile tissue.
3. Glans Penis (enlarged tip): The distal end of the corpus spongiosum, often covered by prepuce (foreskin) if uncircumcised.
Contains three cylindrical bodies of erectile tissue (spongy networks of connective tissue and smooth muscle riddled with vascular spaces that fill with blood during sexual arousal):
- Two corpora cavernosa: Dorsal/posterior in erect position, form most of the root and shaft; engorgement with arterial blood causes the penis to become rigid and erect.
- One corpus spongiosum: Midventral/anterior in erect position, surrounds the urethra and forms the glans penis; its primary function is to keep the urethra open during erection, preventing compression.
Figures for reference: 27.4, 27.6, 27.7.

Accessory Glands:

Contribute Secretions During Sexual Arousal: These glandular secretions mix with sperm and testicular fluid to form semen, providing nourishment, protection, and a transport medium for sperm.
Approximately $95$ % of semen comes from the following glands, listed by contribution:
1. Seminal Vesicles (2):
- Located posterior to the bladder, about $5-7$ cm long, contributing about $60$ % of semen volume.
- Secrete an alkaline fluid containing fructose (a sugar that provides ATP for sperm movement), prostaglandins (hormone-like substances that promote sperm motility and stimulate uterine contractions to draw sperm into the uterus), and coagulating enzymes (e.g., vesiculase) to thicken semen after ejaculation, preventing early dilution.
1. Prostate Gland (1):
- Located inferior to the bladder, encircles the prostatic urethra, about the size of a peach pit, contributing about $25-30$ % of semen volume.
- Secretes a milky, slightly acidic fluid containing citrate (a nutrient for sperm), seminal plasmin (an antibiotic that may destroy bacteria), and prostate-specific antigen (PSA), an enzyme that liquefies coagulated semen after about $20$ minutes, allowing sperm to swim free.
1. Bulbourethral Glands (2):
- Small, pea-sized glands located below the prostate in the urogenital diaphragm (in contact with the external urethral sphincter).
- Secrete a thick, clear alkaline mucus prior to ejaculation, which lubricates the glans penis, neutralizes acidic urine residue in the urethra, and lubricates the path for sperm in the vagina.

C) Spermatogenesis

Sperm Production Within Seminiferous Tubules: A complex process of cell division (mitosis and meiosis) and cell differentiation (spermiogenesis) that takes approximately $64-72$ days.
Process depicted in Figure 27.10.
Begins with spermatogonia ( $2n$ ), which are the stem cells, differentiated into different cells through mitosis and meiosis:
1. Spermatogonia ( $2n$ )- Located at the basement membrane of the seminiferous tubules, these germline stem cells divide by mitosis throughout life to maintain the cell line (producing Type A daughter cells) and produce cells that enter meiosis (Type B daughter cells).
2. Primary Spermatocyte ( $2n$ )- Formed from Type B spermatogonia, these cells move toward the lumen, replicate their DNA, and undergo meiosis I, forming two smaller, haploid ( $n$ ) secondary spermatocytes.
3. Secondary Spermatocytes ( $n$ )- Undergoes meiosis II rapidly, each forming two haploid ( $n$ ) spermatids, resulting in four spermatids from each primary spermatocyte.
4. Spermatids ( $n$ )- Immature sperm cells that undergo spermiogenesis, a process of morphological change lasting about $24$ days, during which they lose most cytoplasm, condense their nuclei, develop flagella, and form acrosomes, ultimately transforming into spermatozoa (mature sperm).

Spermatozoa Structure:

Head:
- Contains a flattened nucleus with $23$ chromosomes ( $n$ ).
- Acrosome (a modified lysosome) covers the top of the nucleus, containing hydrolytic enzymes like hyaluronidase and acrosin to penetrate the secondary oocyte's corona radiata and zona pellucida during fertilization.
Body (Midpiece):
- Contains a large number of tightly packed mitochondria arranged spirally, which produce ATP via aerobic respiration, providing the energy required for movement of the tail.
Tail (Flagellum):
- The locomotor region, propels sperm and is made of microtubules in an organized $9+2$ axoneme arrangement, enabling it to swim through the fluid environment of the male and female reproductive tracts.

Semen Composition:

Consists of: Sperm + testicular fluid ( $5$ %) + accessory gland secretions ( $95$ %), forming a milky white, somewhat sticky mixture.
Volume released at ejaculation: approx. $2-5$ mL, containing $20-150$ million sperm/mL; a sperm count below $20$ million/mL usually indicates male infertility.
pH = $7.2 – 7.6$ (slightly alkaline), providing transportation, nutrients, and protection for sperm by helping to neutralize the hostile acidic environment of the vagina ( $pH = 3.5-4.0$ ), which would otherwise immobilize or kill sperm.

D) Ovarian Reproductive System

Gonads:
- Ovaries are paired almond-shaped organs located in the pelvic cavity on lateral sides of the uterus, connected to the body wall and uterus with ligaments continuous with the peritoneum.
- Histologically, ovaries have four layers:
1. Surface (Germinal) Epithelium:
  - A single layer of cuboidal epithelial cells, representing modified visceral peritoneum lacking connective tissue, that covers the ovary surface.
2. Tunica Albuginea:
  - A dense irregular connective tissue capsule underlying the germinal epithelium, giving the ovary its whitish appearance.
3. Ovarian Cortex:
  - The outer region extending from the tunica albuginea to the medulla, containing ovarian follicles at various stages of development, as well as dense connective tissue and stromal cells.
4. Ovarian Medulla:
  - The inner region containing loose connective tissue, blood and lymph vessels, and nerves, which supply the cortex.

Uterine (Fallopian) Tubes:

Also called oviducts, these tubes are about $10$ cm long and provide a pathway for the ovulated oocyte to reach the uterus; they are the usual site of fertilization.
Composed of three sections:
1. Infundibulum:
- The funnel-shaped, distal end suspended over each ovary with finger-like projections (fimbriae). The fimbriae are ciliated and sweep to create fluid currents, aiding in capturing and moving the ovulated oocyte into the tube.
1. Ampulla:
- The widest and longest middle part, forming about two-thirds of the tube's length, where fertilization usually occurs.
1. Isthmus:
- A narrow, constricted region connecting the ampulla to the uterus.
Histology includes:
1. Mucosa:
- Simple columnar epithelium with ciliated cells to aid in moving the oocyte/zygote towards the uterus (cilia beat towards the uterus) and non-ciliated secretory cells that secrete nutrient-rich fluid (e.g., glycoproteins, lipids, ions) to nourish the oocyte and sperm.
1. Muscularis Externa:
- Smooth muscle layers (inner circular, outer longitudinal) whose rhythmic peristaltic contractions assist in moving the oocyte/zygote towards the uterus.
1. Serosa:
- The outermost layer, a serous membrane derived from the visceral peritoneum.

Uterus:

Hollow, thick-walled, muscular organ located superior to the bladder and anterior to the rectum; about the size and shape of an inverted pear in a non-pregnant female. Its main function is to receive, retain, and nourish a fertilized ovum.
Has three main parts:
1. Fundus:
- The rounded, most superior portion, superior to the entrance of the uterine tubes (isthmus).
1. Body:
- The main central portion, tapering inferiorly, where most embryo implantation and growth occur.
1. Cervix:
- The inferior narrow passageway opening into the vagina; secretes cervical mucus that varies in consistency, playing roles in facilitating sperm transport (when thin) and blocking ascending bacteria (when thick).
Histology of uterus includes three layers:
1. Endometrium:
- The innermost mucous membrane, highly glandular, and composed of:
  - Functional Layer (Stratum Functionalis):
    - The superficial layer developed at puberty, highly vascularized, and responsive to ovarian hormones (estrogen and progesterone); it is shed monthly during menstruation if pregnancy does not occur.
  - Basal Layer (Stratum Basalis):
    - A permanent, deeper layer attached to the myometrium, not shed during menstruation; it undergoes mitosis to replace the functional layer after menstruation, receiving blood from straight arteries.
1. Myometrium:
- The middle muscularis externa, composing the bulk of the uterine wall, composed of interlacing bundles of smooth muscle fibers. It is the thickest layer, especially in the fundus, providing the powerful contractions for childbirth.
1. Perimetrium:
- The outermost layer, a serous membrane and part of the visceral peritoneum.

Vagina:

Birth canal connecting the uterus to the external genitalia, and a female organ of copulation; about $8-10$ cm long, highly distensible.
Histology includes:
1. Mucosa:
- Thick stratified squamous epithelium with transverse folds (rugae), which allows for significant distension during childbirth or intercourse.
- Contains glycogen, which is metabolized by resident bacteria (e.g., Lactobacillus) to lactic acid, creating an acidic environment ( $pH = 3.5-4$ ) that inhibits pathogen growth but is hostile to sperm.
1. Muscularis Externa:
- A smooth muscle layer with inner circular and outer longitudinal fibers, allowing for stretch and contraction.
1. Adventitia:
- The outer fibroelastic connective tissue layer, anchoring the vagina to adjacent organs.

Vulva:

External reproductive structures (pudendum) including:
1. Mons Pubis:
- A fatty, rounded area overlying the pubic symphysis, covered with pubic hair after puberty, cushioning the pubic symphysis during intercourse.
1. Labia Majora/Minora:
- Labia majora are larger, hair-covered, fatty skin folds, homologous to the male scrotum, enclosing the labia minora.
- Labia minora are smaller, hairless folds located medial to the labia majora, which contain sebaceous (oil) glands.
1. Vestibule:
- The central recess or space between the labia minora, containing the vaginal opening (partially covered by a hymen in virgins) and the external urethral orifice, which is anterior to the vaginal opening.
- Contains greater vestibular glands (Bartholin's glands) that secrete mucus to lubricate the vestibule, especially during sexual intercourse.
1. Clitoris:
- A small, sensitive erectile organ, mainly internal with a small external portion (glans clitoris) at the anterior junction of the labia minora; contains two corpora cavernosa, homologous to the male penis but lacking urinary function and serving purely for sensory input and sexual arousal.

E) Oogenesis

Gamete production within ovarian follicles: A complex process involving meiosis and unequal cytokinesis, resulting in one large ovum and discarded polar bodies.
Starts with oogonia ( $2n$ ) in the fetal ovary:
1. Oogonia multiply rapidly by mitosis during fetal development (peaking at $5-7$ million by the fifth month), then differentiate into primary oocytes ( $2n$ ) which enter meiosis I and arrest in prophase I. All primary oocytes are formed before birth (about $1$ million present at birth).
2. At puberty, only about $300,000$ to $400,000$ primary oocytes remain, as most degenerate (atresia) during childhood and adolescence.
3. Each month, typically one primary oocyte (within a dominant follicle) completes meiosis I in response to a surge in Luteinizing Hormone (LH), forming one large secondary oocyte ( $n$ ) and one tiny first polar body ( $n$ ). Unequal cytokinesis ensures the secondary oocyte retains most of the cytoplasm and nutrients.
4. The secondary oocyte begins meiosis II, but arrests in metaphase II, and is then ovulated. If fertilized by a sperm, it completes meiosis II, forming a large ovum ( $n$ ) and a second polar body ( $n$ ).
5. A primary oocyte ultimately produces one functional ovum and 2-3 polar bodies (if the first polar body also divides), which are essentially packets of discarded nuclear material with little cytoplasm, destined to degenerate.

F) Ovarian Follicles & Follicular Development

During various stages of development from before birth to puberty, ovarian follicles mature in a cyclical manner, driven by hormonal changes.
1. Late Embryonic and Fetal Period: Germ cells migrate to developing gonads, differentiate into oogonia, proliferate, and begin differentiating into primary oocytes (arrested in prophase I) within primordial follicles.
2. Childhood: Primordial follicles with primary oocytes begin to degenerate (atresia); the ovary is mostly inactive until puberty, with only about $1$ % of the original follicles remaining.
3. At Puberty: Ovarian cycles begin; primary follicles develop into secondary and vesicular follicles under hormonal influence (FSH):
- Primordial Follicle: Consists of a primary oocyte surrounded by a single layer of flattened follicular cells.
- Primary Follicle: Follicular cells enlarge and become cuboidal (now called granulosa cells) and proliferate to form multiple layers.
- Secondary Follicle: Granulosa cells continue to proliferate, forming multiple layers, and start secreting follicular fluid, which accumulates between the layers. The zona pellucida, a thick glycoprotein layer, forms around the oocyte.
- Vesicular (Antral) Follicle: Fluid-filled spaces within the granulosa layers unite into a single large antrum, creating a mature, dominant follicle. Typically, only one dominant follicle develops per cycle, while others undergo atresia.
- Preovulatory (Graffian) Follicle: Characterized by a large antrum and the oocyte on a stalk of granulosa cells (cumulus oophorus). A surge in hormone levels (specifically Luteinizing Hormone, LH, triggered by high estrogen levels) triggers ovulation, leading to the rupture of the follicle wall and release of the secondary oocyte into the peritoneal cavity.
1. Corpus Luteum:
- An endocrine structure formed from the ruptured follicle remaining post-ovulation. The granulosa cells and internal theca cells enlarge and differentiate into luteal cells, accumulating a yellow pigment (lutein).
- Secretes high levels of estrogen and progesterone to support fetal development by thickening and vascularizing the uterine lining (endometrium) for implantation and maintaining pregnancy if fertilization occurs.
1. Corpus Albicans: A scar tissue that forms if there is no pregnancy; the corpus luteum degenerates into this non-hormone-producing fibrous tissue, signifying the end of the ovarian cycle for that follicle.

G) Mammary Glands

Modified sweat glands (apocrine glands) located within the breasts, anterior to the pectoralis major muscles, responsible for milk production (lactation) in females after childbirth.
Structure and Support:
- Each breast is a hemispheric mound of glandular tissue and adipose (fat) tissue.
- They are anchored to the deep fascia (over the pectoralis major muscles) by suspensory ligaments (Cooper's ligaments), which provide support and maintain the shape of the breast.
- The amount of adipose tissue determines breast size, but has no relation to milk production capacity.
Internal Anatomy:
- Each breast contains $15-25$ small compartments called lobes.
- Within each lobe are smaller units called lobules, which contain the actual milk-producing glands called alveoli (acini).
- Milk secreted from the alveoli passes into lactiferous ducts, which converge to form wider lactiferous sinuses (ampullae) just deep to the areola before opening individually at the nipple.
- Nipple: A pigmented, conical projection containing openings of lactiferous ducts.
- Areola: The circular pigmented skin area surrounding the nipple, containing modified sebaceous glands (Montgomery's glands) that produce an oily secretion to lubricate and protect the nipple and areola during nursing.
Lactation:
- Milk production is stimulated primarily by the hormone prolactin from the anterior pituitary gland.
- Milk ejection (let-down reflex) is stimulated by oxytocin from the posterior pituitary, which causes contraction of myoepithelial cells surrounding the alveoli and ducts, forcing milk towards the nipple.
Development:
- Mammary glands are present