Ch. 28: Reproductive System

Functions of the reproductive system

  • 4 major functions

    1. gametogenesis- production of gametes (sperm and eggs; oocytes)

    2. fertilization- combination of egg and sperm to form a diploid cell

    3. development and nourishment of a new individual- female reproductive system nurtures a developing individual and provides nourishment after birth via breast milk (female-specific)

    4. production of reproductive hormones- reproductive hormones control development of the sex-specific body form


Meiosis

  • meiosis- specialized cell division for producing gametes

  • gametes- reproductive cells with half of the DNA content of non-reproductive cells

  • homologous chromosomes- pairs of chromosomes with the same size/shape/gene sequence, but may have different versions of genes (blue vs green eyes)

  • chromatid- one half of a chromosome

  • human somatic cells have 23 homologous pairs of chromosomes

    • 22 autosomal pairs and 1 sex pair

  • homologous pairs- one chromosome from each parent both containing corresponding genetic information

  • upon fertilization, 2 haploid gametes combine to form a diploid gamete

  • 2 divisions of meiosis:

    • meiosis I:

      • reductive division- reduction of cell chromosomes

      • produces 2 haploid daughter cells with 23 unpaired chromosomes

    • meiosis II:

      • continues with 2 haploid daughter cells with 23 unpaired, double-stranded chromosomes from meiosis I

      • meiosis II produces 4 daughter cells each with 23 chromatids (single-stranded chromosomes)

  • crossing over- exchange of genetic material between maternal and paternal chromosomes

  • independent assortment- random separation of homologous chromosomes during meiosis I


Anatomy of the male reproductive system

  • 4 major components of the male reproductive system:

    1. testes

    2. ducts

      • epididymides (epididymis)

      • ducts deferentia (ductus deferens; nas defeners

      • urethra

    3. accessory glands

      • seminal vesicles

      • prostate glands

      • bulbourethral glands

    4. supporting structures

      • scrotum

      • penis


Scrotum

  • scrotum- saclike structure that contains the testes

    • formed by skin, connective tissue, and a layer of muscles

  • cremaster muscle- extension of skeletal muscles that attach to the testes and control their distance from the body


Perineum

  • perineum- area between the thighs, pubic symphysis, coccyx, and ischial tuberostites

    • 2 areas to the perineum:

      • urogenital triangle- anterior portion containing the base of the penis and scrotum

      • anal triangle- posterior portion containing the anal opening


Testes

  • testes- small, oval-shaped organs within the scrotum

    • function as exocrine and endocrine glands

    • male gonads

  • seminiferous tubules- site of sperm cell formation; contained within the lobules

  • interstitial (Leydig) cells- clusters of endocrine cells near seminiferous tubules which secrete testosterone


Spermatogenesis

  • spermatogenesis- development of sperm cells

  • 2 cells within seminiferous tubules are important for sperm formation:

    • germ cells- cells that undergo meiosis to form 4 haploid daughter cells

    • sustentacular (Sertoli) cells- nourish germ cells and produce some hormones (not testosterone)

      • sex hormone binding globulin- protein secreted by sustentacular cells to increate sex hormones half-life

  • spermatogonia (spermatogonium)- germ cells undergoing mitotic division

  • spermatids- 4 haploid daughter cells produced from a single primary spermatocyte

    • produced by second meiotic division; contain 23 chromatids (single-stranded chromosomes)

  • spermiogenesis- last phase of spermatogenesis that produces a sperm cell

    • spermatozoon (spermatozoa)- sperm cells

  • during spermiogenesis, each spermatic develops 3 structures

    • head- contain acrosome

      • acrosome- specialized vesicle with enzymes that allow the sperm cell to penetrate the oocyte (egg)

    • midpeice

    • flagellum (tail)- allows sperm cell self-propelled motility


Ducts

  • epididymis- comma-shaped structures on the posterior aspect of each testis

    • contain cilia to help move sperm along ducts

  • final sperm cell maturation occurs in the epididymis

  • ductus deferens- connects epididymus to ejaculatory duct

  • ampulla- enlargement of the deferens where it approaches the ejaculatory duct near the prostate


Urethra

  • male urethra has 3 sections:

    1. prostatic urethra- connected to the bladder; passes through the prostate gland

    2. membranous urethra- shortest portion that connects prostatic and spongy urethra

    3. spongy urethra- longest portion; opens into eternal urethral orifice


Penis

  • penis- male organ of copulation

  • copulation- transfer of sperm cells from male to female for fertilization

  • composed of 3 columns of erectile tissue

    • corpus cavernosa (2)- 2 large columns forming the majority of erectile tissue

    • corpus spongiosum (1)- 1 column surrounding the urethra and extends to form the head of the penis

  • glans penis- distal end of the penis formed by corpus spongiosum


Accessory glands

  • 3 major glands required for normal reproductive function

    • seminal vesicles- sac-shaped glands near the ampullae and prostate

    • prostate gland- walnut-shaped/sized dorsal to the pubic symphysis

    • bulbourethral glands- pair of pea-sized glands near the membranous urethra


Semen

  • semen- collective secretions from seminiferous tubules in testes and accessory glands

  • seminal vesicles- secrete thick fluid with 3 major functions:

    • Nourish sperm cells

    • Coagulate semen

    • Movement of sperm cells

      • enzymes to help sperm reach female reproductive tract via uterine contractions


Regulation of reproductive hormone secretion

  • hormones are responsible for:

    • development of reproductive structures

    • maintenance of reproductive functions

      • sperm cell formation

    • development of secondary sexual characteristics

  • neural mechanisms are responsible for:

    • sexual behavior

    • control of the sex act

  • 3 important hormonal mechanisms/systems influence male reproductive system:

    • hypothalamus

      • releases gonadotrophin-releasing hormone (GnRH)

    • pituitary gland

      • releases 2 gonadotrophin hormones in response to GnRH

        • luteinizing hormone (LH)- stimulated testosterone production/release

        • follicle-stimulating hormone (FSH)- stimulated sperm cell development

    • testes (male gonads)

      • releases testosterone in response to LH


Effects of testosterone

  • wide range of testosterone effects:

    • enlargement of genitals and development of the duct system

    • development of secondary sexual characteristics

      • growth of thicker, coarser, pigmented hair in the pubic area, axillary region, face, and entire body

      • increased sebaceous gland production, especially in the face

      • hypertrophy of the larynx

      • increased metabolic rate (ex, protein synthesis), red blood cell production, blood volume


Male sexual behavior and the male sex act

  • male sex act involved a sequence of 4 major reflexes

    • erection of the penis

    • secretion of mucus into the urethra

    • emission- discharge of secretions from the ducts deferential into the urethra

    • ejaculation- forceful expulsion of semen from the urethra

      • caused by contractions of urethra, pelvic flood, and muscles at the base of the penis

  • parasympathetic action potentials induce bulbourethral secretion

  • sympathetic centers control emission and ejaculation

    • induces seminal vesicle and prostate gland secretion

    • induces peristaltic contractions of reproductive ducts

  • somatic motor action potentials control ejaculation

    • induce rhythmic contractions of skeletal muscles of the pelvic floor and base of penis

  • sex act = parasympathetic + sympathetic + somatic


Anatomy of the female reproductive system

  • female reproductive system consists of 6 major structures

    • ovaries

    • uterine tubes

    • uterus

    • vagina

    • external genital organs

    • mammary glands


Ovaries

  • ovaries- female gonads

  • supported by 3 structures

    • mesovarium- peritoneal fold anchoring the ovaries to the broad ligament

    • suspensory ligament- anchors mesovarium to the abdominal wall

      • carries ovarian arteries, veins, and nerves

    • ovarian ligament- anchors the ovary to the superior aspect of the uterus

  • ovarian follicles- structures containing developing primary oocytes

    • distributed throughout the cortex


Oogenesis and fertilization

  • oogenesis- process of female gamete production

    • begins before birth

    • ovaries contain ~5 million oocyte germ cells (oogonia) 4 months following fertilization

  • primary oocytes- diploid female sex cells that undergo meiosis I

  • female meiosis results in 2 uneven cells

    • secondary oocyte- larger cell created by meiosis I

      • also called “ovum”

    • first polar body- smaller cell with less cytoplasm

  • ovulation- release of a secondary oocyte from an ovary

  • meiosis II is only completed if the secondary oocyte is fertilized

  • fertilization-- process of sperm binding to and penetrating a secondary oocyte

  • zygote- fertilized ovum with 23 pairs of homologous chromosomes


Follicle development

  • ovarian follicles mature while their oocytes are undergoing meiosis

  • ovarian cycle- the reoccurring events taking place in the ovaries of sexually mature, non-pregnant feales

  • primordial follicle- premature follicle present at birth

    • granulosa cells- single layer of flat cells surrounding primary follicles

  • primary follicles- enlarged follicles with enlarged oocytes, a ticker granulosa cell layer, and a zona pellucida

  • every ~28 days, hormonal changed stimulate development of a secondary follicle

  • secondary follicle- contains oocyte, granulosa cell layer, vesicles, and the theca

    • vesicles- fluid filled spaces

    • theca- capsule surrounding the follicle

  • mature follicle- contains an antrum, oocyte, and cumulus cells

  • mature follicle forms a superficial lump on the ovary

    • ruptures during ovulation

    • releases oocyte surrounded by cumulus cells into peritoneal cavity

    • usually only one mature follicle goes through ovulation

  • corpus luteum- endocrine structure formed by the ruptured mature follicle   

    • remaining granulosa and theca cells become luteal cells

  • luteal cells- secrete progesterone and estrogen

  • if pregnancy occurs- corpus luteum enlarged and remains active

  • it no pregnancy occurs- corpus luteum functional for ~10 days before degenerating


Uterine tubes

  • uterine (fallopian) tubes- tubes extending form each ovary to the uterus

  • no directly attached to the ovary

  • infundibulum- funnel-like expansion of the uterine tube near the ovary that receives oocyte during ovulation

  • ampulla- widest, longest part of the tube nearest the Infundibulum

    • usually where fertilization occurs


Uterus

  • uterus- site of implantation following fertilization

    • size and shape of a medium pear

  • cervix- smaller, narrow, inferior portion of the uterus

  • 3 ligaments support the uterus:

    1. broad ligament- pereitoneal fold anchoring the lateral uterus to the wall of the pelvis

      • also supports ovaries and uterine tubes

    2. round ligaments- anchor the lateral uterus to the labia majora

    3. uterosacral ligament- anchor the lateral uterus to the sacrum

  • pelvic flood muscles support the uterus inferiorly

  • uterus wall has 3 layers:

    1. perimetrium- visceral peritoneum covering the uterus

    2. myometrium- muscular layer forming the bulk of the utereus

    3. endometrium- innermost layer inning the uterus; 2 sub-layers    

      1. basal layer- deepest part continuous with myometrium

      2. functional layer- undergoes changes and sloughing during the uterine cycle

        • supplied by small spiral arteries


Vagina

  • vagina- female organ of copulation extending from the uterus to the external environment

  • fornix- superior, domed section of the vagina that attaches to the cervix


Female external genitalia

  • vulva- female external genitalia

  • labia minora- pair of labia minora come together superior to the vestibule

  • clitoris- small, erectile structure located just inferior to the prepuce

    • contains paired corpora cavernosa

  • bulbs of the vestibule- erectile tissue edeep to the vestibular wall and surrounding the vagina

    • corresponds to corpus spongosum

  • greater vestibular glands- secrete lubricating fluid between the vaginal opening and the labia minora (FEMALE GLAND)

  • lesser vestibular glands- secrete lubricating fluid near the clitoris and urethra opening (FEMALE GLAND)

  • labia majora- paired, prominent, rounded folds of skin lateral to the labia minora


Perineum

  • bounded by the same landmarks as the male perineum; same 2 triangle regions

  • clinical perineum- region between the vagina and anus


Mammary glands

  • mammary glands- organs of milk production within the breasts

  • lobes- components of mammary glands containing milk producing and collecting structures

  • lactiferous duct- milk-producing duct

  • lactiferous sinus- enlargements of lactiferous ducts near the nipple where milk collects during milk letdown

  • alveoli- expanded ends of small lactiferous ducts where milk is produced and expelled from lobules


Puberty in females

  • estrogen and progesterone are the primary female reproductive hormones

    • estrogen refers to multiple hormones, including estradiol

    • menarche- first episode of menstrual bleeding near the end of puberty

  • female reproductive hormone mechanisms are similar to male hormones

  • LH and FSH stimulate ovaries to secrete progesterone and estrogen


Menstrual cycle

  • menstrual cycle often divided into 2 components

  • ovarian cycle has 2 phases of roughly equal length:

    • follicular phase- occurs before ovulation

      • mature follicle develops from a primordial follicle

    • luteal phase- begins immediately after ovulation

      • corpus luteum forms and produces progesterone and estrogen and degenerated if there is no pregnancy

  • uterine cycle has 3 phases:

    1. menses-

      • period of milk hemorrhage when the endometrium is sloughed and expelled from the uterus

      • stimulated by declining progesterone concentrations during corpus luteum degradation

    2. proliferation phase-

      • time between menses and ovulation

      • ends with ovulation

    3. secretory phase-

      • time between ovulation and the next menses

      • prepares endometrium to receive and support a fertilized oocyte


Female sexual behavior and the female sex act

  • neural inputs control female reproductive reflexes


Female fertility and pregnancy

  • sperm cell motility and female reproductive contractions help sperm travel to the ampulla

  • ampulla- site of fertilization

  • trophoblast- developing embryo created by fertilization

  • embryo migrates from the ampulla to the uterus

  • placenta- organ of nutrient and waste exchange between embryo and mother

    • formed during trophoblast implantation

  • ovaries and placenta are important hormonal regulators of pregnancy

  • human chorionic gonadotropin (hCG)- released from the placenta to stabilize the corpus luteum and prevent degradation

  • ovaries release estrogen and progesterone during early pregnancy

  • mild pregnancy, placenta secretion of estrogen and progesterone takes over


Menopause

  • menopause- cessation of the menstrual cycles in older females

  • associated with age-related ovarial changes

    • number of follicles is depleted, less estrogen/progesterone produced

    • remaining follicles are less sensitive to LH and FSH