Intrauterine Development Biology

Intrauterine Development

• Testosterone presence in a male fetus causes the mesonephric duct to develop into male reproductive organs and the paramesonephric duct regress.
• The absence of testosterone in a female fetus at week 10 causes the gonadal tissue to differentiate into ovaries and the paramesonephric duct develops into the female reproductive organs.
• All oocytes are formed in the ovaries by week 10 of intrauterine development.
• At week 12, the external genitalia develops; in males, testosterone causes the penile tissue to elongate and the urogenital fold closes, forming the urethra.
• In females, the absence of testosterone causes the urogenital fold to remain open, forming the labia minora. The scrotal tissue in a male develops into the labia majora in a female.
• If testosterone is halted in a male fetus it results in a chromosomal male with female-appearing genitalia.
• Testes that remain in the pelvic cavity may not produce viable sperm and are associated with testicular cancer.

Male Reproductive System: External Structures (Testes)

• Sperm production starts when the hypothalamus releases GnRH, which influences the APG to release FSH and LH.
• FSH releases androgen-binding protein and LH releases testosterone, which promotes sperm formation.
• As testosterone increases, a feedback effect is created on the hypothalamus and APG which slows down the production of FSH and LH, thereby regulating sperm production.
• One testis is usually larger than the other and is suspended slightly lower in the scrotum (usually the left one).
• Spermatozoa do not survive at temperatures as high as that of the body.
• The location of the testes outside of the body is 1 degree F lower than body temperature.
• Normal testes are firm, smooth, and egg-shaped.

Male Reproductive System: External Structures (Penis)

• The penis is composed of three cylindrical masses of erectile tissue: two called the corpus cavernosa and one called the corpus spongiosum.
• The penis serves as an outlet for both the urinary and reproductive tracts.
• The urethra passes through the erectile tissue of the penis.
• With sexual excitement, nitric oxide is released from the endothelium of blood vessels, leading to increased blood flow to the penis (engorgement).
• The ischiocavernosus muscle at the base of the penis contracts, trapping blood in the erectile tissue, leading to distention of the penis and subsequent erection.

Female External Structures

• The vulva is highly sensitive to touch, pressure, pain, and temperature due to a rich nerve supply.
• The anterior portion of the vulva is derived from the ilioinguinal and genitofemoral nerves (L1 level).
• The posterior portion of the vulva is derived from the pudendal nerve (S3 level).
• Normal stretching of the perineum during childbirth causes temporary loss of sensation in the area.

Female Internal Structures: Ovaries

• The ovaries are located near the uterus in the lower abdomen.
• They are almond-shaped, grayish-white, and approximately 4 cm long, 2 cm in diameter, and 1.5 cm thick.
• The ovaries function to produce, mature, and discharge ova (egg cells) and produce estrogen and progesterone.
• They initiate and regulate menstrual cycles.
• After menopause, the uterus, breasts, and ovaries undergo atrophy due to a lack of estrogen.

Female Internal Structures: Uterus

• The uterine wall is composed of three layers; the endometrium (inner), myometrium (middle), and perimetrium (outer).
• The endometrium is responsible for menstrual function and is highly sensitive to estrogen and progesterone.
• The endocervix is the mucous lining of the cervix and secretes mucus to lubricate the surface and allow sperm to pass through.
• The myometrium, or muscle layer, is composed of smooth muscle fibers that offer strength to the organ and prevent regurgitation of menstrual blood into the tubes.
• The myometrium also holds the internal cervical os closed during pregnancy to prevent preterm birth.
• The myometrium has an equal pressure to expel the fetus due to its unique muscle fiber arrangement.
• The perimetrium offers additional support and strength to the uterine structure.

Uterine Blood Supply

• The descending abdominal aorta divides into the two iliac arteries, which further subdivide to form the hypogastric arteries, supplying blood to the uterus.
• The ovarian artery (direct subdivision of the aorta) joins the uterine artery as a fail-safe system to ensure adequate blood supply to the uterus.