Reproductive System

Fetal Development and Differentiation

• Fetal differentiation is categorized in three different ways.
• Genetic sex is determined at conception: Male XY, Female XX.
• Gonadal sex is a series of events where sexually indifferent gonads and genitalia progressively acquire male or female characteristics.
• Specific genes induce gonadal differentiation around week 5-7.
• The SRY gene on the Y chromosome drives testes development.
• Absence of SRY will develop ovaries at approximately 9 weeks.
• Internal and external genitalia will follow the male pathway in the presence of specific testicular hormones.
• The female pathway will progress when these hormones are not present.
• In testes, germ cells produce spermatogonia; Sertoli cells synthesize antimüllerian hormone; Leydig cells synthesize testosterone.
• Ovaries contain germ cells that produce oogonia, which remain in the primary oocyte stage of meiosis until puberty.
• Granulosa & Theca cells work together to control hormone levels in the follicle and bloodstream, synthesizing progesterone and estradiol.
• Phenotypic sex is the outward appearance of maleness & femaleness that develops in response to gonadal hormones.
• Two major differences in male/female phenotype development include testosterone and anti-Müllerian hormone.
• Testosterone & anti-Müllerian hormones are typically produced in the testes.
• In their presence, male genitalia develop, and without them, female genitalia develop.
• Females have absence of androgens and presence of estrogen.
• Anti-müllerian hormone & testosterone are needed for the male phenotype.
• Testosterone stimulates growth & differentiation of wolffian ducts, while antimüllerian hormone causes atrophy of the müllerian ducts.
• The internal genital tract includes the prostate, seminal vesicles, vas deferens, & epididymis.
• Wolffian ducts give rise to the epididymis, vas deferens, seminal vesicles, & ejaculatory ducts.
• The external genitalia includes the scrotum & penis, differentiating at gestational weeks 9-10.
• Differentiation depends on testosterone conversion to dihydrotestosterone & presence of androgen receptors on target tissue.
• Ovaries secrete estrogen, but neither antimüllerian hormone nor testosterone.
• No testosterone is available to stimulate growth & differentiation of wolffian ducts, and no antimüllerian hormone is available to suppress differentiation of müllerian ducts.
• Müllerian ducts develop into the internal female tract.
• Internal genitalia include the fallopian tubes, uterus, and upper 1/3 of the vagina.
• External genitalia include the clitoris, labia majora & minora, and lower 2/3 of the vagina.
• Estrogen is needed for developing external female genitalia to normal size.

Gonadotropin Secretion

• Both Males and Females release GnRH from the hypothalamus, triggering the anterior pituitary to release FSH & LH.

Events at Puberty

• Pulsatile secretion of GnRH drives a pulsatile secretion of FSH and LH, where sensitivity to GnRH receptors increases in anterior pituitary.
• The onset of the maturational process at puberty is genetically programmed.

Male Hormones

• FSH, secreted by the anterior pituitary, initiates sperm production in the testes.
• LH, secreted by the anterior pituitary, stimulates testosterone secretion by the testes.
• Inhibin, secreted by testes, decreases the secretion of FSH to maintain a constant rate of spermatogenesis.
• Testosterone, secreted by the testes, promotes sperm maturation.
• Testosterone initiates the development of secondary sex characteristics, including growth of the reproductive organs, larynx, and facial/body hair.
• Testosterone also stimulates increased protein synthesis, especially in skeletal muscles.
• Testosterone mediates differentiation of the epididymis, vas deferens, and seminal vesicles; increased muscle mass; pubertal growth spurt; epiphyseal closure; growth of the penis and seminal vesicles; deepening of voice; spermatogenesis; negative feedback on the anterior pituitary; and libido.
• Dihydrotestosterone mediates differentiation of the penis, scrotum, and prostate; male hair pattern; male pattern baldness; sebaceous gland activity; and prostate growth.

Negative Feedback in Males

• Hypothalamus releases GnRH which stimulates the anterior pituitary.
• The anterior pituitary then releases FSH and LH.
• FSH and LH release stimulates Sertoli cells and Leydig cells located in the testes.
• The Sertoli and Leydig cells then release testosterone.
• In return, testosterone regulates hypothalamus/anterior pituitary activity.

Meiosis

• Complex cell division in sexually reproductive organisms.
• Two rounds of cell division result in haploid gametes, which are half the diploid number.
• One cell with diploid (46) divides to 4 cells with haploid.
• Gametes are sperm or egg cells.
• Germ cells give rise to gametes.
• Oogenesis is meiosis that takes place in the ovaries.
• Spermatogenesis is meiosis that takes place in the testes.

Spermatogenesis

• The process of meiosis in the testes, the site of sperm production; FSH and Testosterone are needed.
• Spermatogonium Division involves division by mitosis to form a stem cell and a primary spermatocyte.
• Hormonal Regulation occurs through GnRH, which stimulates the pituitary to secrete FSH and LH to regulate sperm production.
• Sperm Production begins at puberty & continues throughout life, but diminishes with age.
• Seminiferous Tubules contain spermatogonia, stem cells that generate sperm.

Oogenesis

• The formation of a egg cell through meiosis that is regulated by hormones after puberty.
• Ovarian follicle development begins before birth, each containing an oogonium that becomes a primary oocyte.
• Meiosis I Prophase and Quiescence is the phase where primary oocytes enter meiosis I prophase, then become quiescent until puberty.
• During the cycle of Puberty and menstruation, GnRH and FSH initiate follicle growth and estrogen production, and one oocyte completes meiosis I.
• In Meiosis II and Fertilization, the secondary oocyte reaches metaphase II, completing meiosis II upon fertilization.
• Egg Production begins at puberty and continues until menopause (30-40 years), producing one mature ovum each month.
• Meiosis and Chromosome Maintenance reduce chromosome number by half, maintaining the diploid state in gametes.

Male Reproductive System

• Testes (paired) are responsible for producing sperm and testosterone and are located in the scrotum.
• Seminiferous tubules, the site of sperm production, are contained in the testes with a sperm production-specific temperature of 96 degrees.
• The epididymis (paired) stores and matures sperm before they are ejaculated.
• Ductus deferens (paired) transports mature sperm from the epididymis to the ejaculatory ducts via peristalsis.
• Ejaculatory ducts (paired) carry sperm and seminal fluid from the ductus deferens and seminal vesicles to the urethra.
• Seminal vesicles (paired) produce an alkaline fluid that mixes with sperm to form semen
• Contains fructose for ATP production.
• The prostate gland produces a citric acid fluid that helps nourish and protect sperm.
• Smooth muscle aids forced ejaculation.
• Bulbourethral glands (paired) located below the prostate gland empty into the urethra.
• Secretion is alkaline and helps to line the urethra before ejaculation to increase sperm motility.
• Semen is composed of sperm and secretions from the seminal vesicles, prostate, and bulbourethral glands.
• The alkaline pH of semen neutralizes the acidic pH of the female vagina.
• The urethra carries semen to the exterior, within the penis.

The Female Reproductive System

• The ovaries (paired) produce and release eggs for potential fertilization via LH stimulation.
• Fallopian tubes (paired) transport the egg from the ovary to the uterus.
• Fertilization usually occurs.
• Fimbriae sweep the ovum into the tube.
• Egg is moved by a combination of peristalsis and ciliated epithelial tissue.
• The uterus provides a nourishing environment for the developing fetus during pregnancy.
• The myometrium is the smooth muscle layer, which contracts for delivery.
• The endometrium lines the uterus, which may become the placenta.
• The basilar layer of the endometrium is permanent, while the functional layer is shed and regenerated during menstruation.
• The vagina is the birth canal that allows for sexual intercourse and the passage of a baby during labor.
• The vagina extends from the cervix to the vaginal orifice.
• The normal vaginal flora provides an acidic pH that inhibits pathogen growth.
• External genitals consist of the clitoris (small mass of erectile tissue), labia major and minor (folds of skin), and Bartholin's glands (open to vaginal orifice - secretes mucus).
• Mammary glands produce and secrete milk for the nourishment of offspring.

Female Hormones

• Estrogen is secreted by the ovary and placenta during pregnancy.
• Functions of estrogen include promoting maturation of ovarian follicles, promoting blood vessel growth in the endometrium, and initiating the development of secondary sex characteristics.
• Examples of secondary sex characteristics are growth of the uterus & other reproductive organs, growth of mammary ducts, fat deposition in breast tissue, broadening of the pelvic bone, and subcutaneous fat deposition in hips and thighs.
• Progesterone is secreted by the ovary (corpus luteum) during pregnancy and the placenta during pregnancy.
• Promotes successful implantation of an embryo in the endometrium.
• Promotes growth of blood vessels in the endometrium & storage of nutrients.
• Inhibits contractions of the myometrium.
• Inhibin, secreted by the ovary (corpus luteum), inhibits secretion of FSH.
• Relaxin is secreted by the ovary and it inhibits the contractions of myometrium to facilitate implantation.

Hormone Control in Females

• Hormone control varies during the menstrual cycle, where follicular and luteal phases have negative feedback control.
• Midcycle is characterized by the positive control feedback of estradiol.

Aging and the Reproductive System

• In females, aging is a definitive end to reproduction.
• Menopause indicates an end to ovulation and menstrual cycles.
• Perimenopause is the stage preceding menopause
• Can start as early as 35
• Characterized by irregular menses, hot flashes, and mood changes.
• Menopause usually starts between ages 45 and 55.
• Effects of decreased estrogen include loss of bone mineral density, increased blood cholesterol, drying of the vaginal mucosa, more susceptible to infection, urinary incontinence, weight gain, headaches, and mood change.
• Males have testosterone secretion that continues throughout life, as does sperm production.
• Both may diminish with age.
• Prostatic hypertrophy involves enlargement of the prostate gland.
• As the urethra is compressed by the growing prostate gland, urination becomes difficult, and residual urine in the bladder increases the chance of urinary tract infection.
• Cancer of the prostate is one of the more common cancers in elderly men.