Lecture 10 - Reproductive System

The male reproductive system's primary purpose is to produce and supply sperm. Its external anatomy includes the penis and scrotum, with the testes, the site of sperm production, located within the scrotum.

The scrotum houses the testes, which contain seminiferous tubules organized into lobes. These tubules contain Leydig cells, producing testosterone, and Sertoli cells, producing androgen-binding protein (ABP). The combination of testosterone and ABP promotes sperm production.

The anterior pituitary gland, stimulated by the hypothalamus, releases gonadotropins: Luteinizing Hormone (LH) and Follicle-Stimulating Hormone (FSH). LH stimulates Leydig cells to produce testosterone, while FSH stimulates Sertoli cells, initiating sperm production at puberty.

Sperm produced in the testes matures and is stored in the epididymis before traveling through the vas deferens. The vas deferens joins with the seminal vesicles, which contribute fructose-rich fluid to semen. This mixture enters the ejaculatory duct and then the urethra, passing through the prostate gland, which also contributes fluids.

Erection is mediated by the parasympathetic nervous system, dilating blood vessels and trapping blood in the penis. Ejaculation is controlled by the sympathetic nervous system, causing smooth muscle contractions to expel semen.

The female reproductive system begins with the external genitalia, or vulva, including the mons pubis, labia majora, labia minora, and clitoris (a highly sensitive organ for sexual arousal). The vestibule contains the external urethral orifice and the vaginal orifice.

The vagina has an acidic pH to protect against infection. It leads to the cervix, the 'neck' of the uterus. The uterus is a muscular organ with an inner endometrium (thickens during pregnancy) and a thick myometrium (contracts during childbirth). The uterine tubes (fallopian tubes) connect the uterus to the ovaries. The ampulla of the fallopian tube is the most common site of fertilization. The fimbrae, finger-like projections at the end of the fallopian tubes, help guide the egg into the tube after ovulation from the ovary.

The ovaries contain and release eggs (oocytes) and produce key hormones like progesterone and estrogen, essential for maintaining the endometrial lining of the uterus.

The female reproductive cycle, typically 28 days, begins with primordial follicles (containing oocytes) maturing into primary follicles. Follicle-Stimulating Hormone (FSH) from the anterior pituitary gland stimulates primary follicles to become secondary follicles. These secondary follicles develop granular and theca cells, which produce estrogen.

Estrogen prepares the uterine lining for implantation. High estrogen levels cause a positive feedback loop, leading to a surge in LH and FSH. The LH surge is crucial for ovulation, causing the mature (Graafian) follicle to release the egg around day 14.

After ovulation, the remaining follicular cells form the corpus luteum, stimulated by LH. The corpus luteum produces progesterone, which further thickens and vascularizes the endometrial lining. If fertilization doesn't occur, the corpus luteum degenerates after about 10 days, leading to a drop in progesterone and estrogen, causing menstruation (shedding of the endometrial lining) around day 28.

The testes produce sperm and androgens like testosterone. They are housed in the scrotum, exterior to the body, for optimal temperature regulation (2-3 degrees cooler than body temperature). The spermatic cord contains the ductus deferens (sperm transport), testicular artery (blood supply), pampiniform plexus (cools arterial blood), and cremaster muscle (raises/lowers testes). The dartos muscle wrinkles the scrotum for further temperature control.

Within the testes, sperm is produced in seminiferous tubules. These tubules contain Sertoli cells (sustentacular cells), which produce androgen-binding protein (ABP), and Leydig cells (interstitial cells), located outside the tubules, which produce testosterone. Stem cells called spermatogonia in the tubule walls differentiate into sperm.

From puberty, the hypothalamus releases gonadotropin-releasing hormone, stimulating the anterior pituitary to release FSH and LH. FSH acts on Sertoli cells, and LH acts on Leydig cells. Together, the ABP from Sertoli cells and testosterone from Leydig cells stimulate spermatogenesis (sperm production) from spermatogonia.

Gonadotropins (FSH and LH) are produced by the anterior pituitary gland and regulate both male and female reproduction. In females, FSH stimulates follicle development and estrogen production, while LH triggers ovulation and progesterone production by the corpus luteum, preparing the uterus for implantation. In males, FSH stimulates Sertoli cells to produce ABP, and LH stimulates Leydig cells to produce testosterone; these together drive sperm production.