Human Reproductive System: MDCAT Biology notes

Male reproductive system

• Scrotum: holds the testes outside the abdomen and keeps them 2–3 °C below body temperature (about 35 °C). This is the best temperature for sperm production.
• Testis: divided into about 200–300 lobules, each packed with coiled seminiferous tubules, where spermatogenesis takes place.
• Sertoli cells line the seminiferous tubules. Their fluid gives the sperms a liquid medium, protection and nourishment. They also secrete inhibin, which regulates the rate of spermatogenesis.
• Leydig (interstitial) cells lie between the tubules and secrete testosterone. Testosterone drives sperm production and male secondary sexual characters, such as pubic hair and broad shoulders at puberty.

Path of sperm

Seminiferous tubules → vasa efferentia (these carry sperm out of the testis) → epididymis (stores and matures sperm, then transports it onward) → vas deferens (sperm duct) → urethra. The urethra is the urinogenital duct, carrying both urine and semen. It lies outside the testis. The spermatic cord (vas deferens with blood vessels and nerves) and the spermatic duct are different structures.

Accessory glands

• Seminal vesicles: add fructose-rich fluid and prostaglandins, which help transport sperm.
• Prostate gland: adds alkaline fluid.
• Cowper's (bulbourethral) glands:neutralize the acidity of urine left in the urethra and lubricate it.

Semen is sperm plus these fluids.

Spermatogenesis

Spermatogonia (2n, undifferentiated germ-line cells) divide by mitosis and grow into primary spermatocytes (2n). These undergo meiosis I to give secondary spermatocytes (n, haploid), then meiosis II to give spermatids (n), which mature into sperms. The acrosome in the sperm head releases hyaluronidase, which lets the sperm penetrate the ovum. Gametes are formed by meiosis.

Female reproductive system

• Ovaries: make ova and the hormones oestrogen and progesterone. Oogenesis produces the secondary oocyte by reduction division (meiosis I). Meiosis II is completed only when a sperm fertilizes the oocyte.
• Oviduct (fallopian tube): carries the ovum toward the uterus. Its ciliated cells move the ovum, and its non-ciliated cells moisten and nourish it. Fertilization happens in the proximal part of the oviduct.
• Uterus: a small, pear-shaped, muscular, distensible organ. It receives, retains and nourishes the embryo, and gestation takes place here. The zygote implants roughly 6–7 days after fertilization. The uterus opens into the vagina through the cervix, which is part of the uterus.
• Vagina: the birth canal.

Key hormones

• LH, also called ICSH (interstitial cell stimulating hormone) in males, stimulates the Leydig cells. FSH stimulates the Sertoli cells. Both come from the pituitary, and pituitary failure can stop ovulation.
• Oxytocin acts on the myometrium to cause labour contractions, and on the mammary glands to eject milk.
• After menopause the ovaries stop producing significant oestrogen.

The embryo develops inside the amniotic sac. In in vitro fertilization (IVF), eggs are fertilized in the laboratory and the embryo is placed in the uterus. IVF is used when sperm are too weak to fertilize the egg in the oviduct.

Common MDCAT traps

• Leydig cells make testosterone; Sertoli cells nourish sperm.
• Fertilization happens in the oviduct. Implantation happens in the uterus.
• Spermatogonia become primary spermatocytes next. Spermatogonia are diploid, not haploid.
• Hyaluronidase lets the sperm penetrate the ovum.
• The cervix belongs to the female; seminiferous tubules belong to the male.

Quick revision

• ICSH is the same hormone as LH.
• The scrotum regulates testis temperature.
• The myometrium is the contractile layer of the uterus.
• Cowper's glands neutralize urethral acidity.
• Secondary spermatocytes are haploid.