How the male and female reproductive systems make gametes and new life

Follow a sperm cell's path out of the body:

1. Testes: sperm are made inside coiled seminiferous tubules. The testes sit in the scrotum because sperm production works best 2 to 4°C below core body temperature.
2. Epididymis: a coiled tube on each testis where new sperm continue to mature.
3. Ductus (vas) deferens: a muscular tube that carries sperm out of the scrotum and up into the pelvis.
4. Ejaculatory duct: where the ductus deferens joins the duct of the seminal vesicle.
5. Urethra: runs through the prostate and the penis and carries semen out of the body.

Three accessory glands make most of the semen. The seminal vesicles make fluid rich in fructose, which sperm use to make ATP. The prostate gland surrounds the urethra at the base of the bladder. The bulbourethral glands make lubricating fluid.

• Ovaries: the female gonads. They hold the developing oocytes in follicles and secrete estrogen and progesterone.
• Uterine (fallopian) tubes: carry the oocyte toward the uterus. The open end near the ovary is fringed with finger-like fimbriae. Around ovulation, sweeping muscle contractions and cilia beating toward the uterus draw the oocyte in and along. The wider middle region, the ampulla, is where fertilization most often happens.
• Uterus: the muscular organ where an embryo implants and grows. Its thick smooth-muscle middle layer is the myometrium. Its inner lining, the endometrium, has a deeper stratum basalis that stays and a stratum functionalis that builds up each cycle and is shed during menstruation.
• Cervix: the narrow lower part of the uterus that opens into the vagina.
• Vagina: the muscular canal leading from the cervix to the outside of the body.

Both processes start with diploid cells (46 chromosomes) and use meiosis to make haploid gametes (23 chromosomes).

Spermatogenesis happens in the seminiferous tubules. It begins at puberty and continues for life. Spermatogonia divide by mitosis to produce primary spermatocytes, and meiosis turns each one into four haploid spermatids, which develop into sperm. Sertoli cells wrap around the developing cells and support them. A sperm has a head (nucleus plus an enzyme-filled cap, the acrosome), a mid-piece full of mitochondria, and a tail (flagellum).

Oogenesis starts before birth, when primary oocytes form in the fetal ovary. Each cycle, the LH surge restarts meiosis in one of them, and the resulting secondary oocyte is released at ovulation. The divisions are unequal: one cell keeps almost all the cytoplasm, and the small polar bodies break down. The second meiotic division finishes only if a sperm penetrates the oocyte.

The hypothalamus releases GnRH, which tells the anterior pituitary to release FSH and LH.

Most of this is negative feedback: testosterone, estrogen, and progesterone usually hold back GnRH, LH, and FSH. The one big exception is just before ovulation, when very high estrogen from the dominant follicle flips to positive feedback and causes the LH surge.

The ovarian cycle (the follicle) and the menstrual cycle (the uterine lining) run side by side. Day 1 is the first day of bleeding.

1. Menses (about days 1–5): progesterone is low, so the stratum functionalis is shed.
2. Follicular phase / proliferative phase: FSH and LH make follicles grow and secrete estrogen. Rising estrogen rebuilds the endometrium.
3. Ovulation (about day 14): very high estrogen triggers the LH surge, and the dominant follicle releases its secondary oocyte.
4. Luteal phase / secretory phase: the empty follicle becomes the corpus luteum and secretes progesterone. The endometrial glands secrete a nutrient-rich fluid, readying the lining for implantation.
5. No pregnancy: after about 10 to 12 days, the corpus luteum breaks down. Progesterone and estrogen fall, the lining is shed, and a new cycle begins.

Fertilization usually happens in the uterine tube, most often the ampulla. An unfertilized oocyte survives only about 24 hours, too short for the roughly 72-hour trip to the uterus. Sperm can survive in the tubes for 3 to 5 days.

When sperm reach the oocyte, their acrosomes release enzymes that clear a path through its outer layers, including the zona pellucida, until one sperm fuses with the oocyte. The two haploid nuclei combine to form a diploid zygote.