Passing the Torch: Anatomy and Physiology of Human Reproduction

Claire Brooks

Here's a strange fact to start with. Your heart, your lungs, your kidneys, your muscles, they all work minute by minute to keep you, one individual, alive. The reproductive system does none of that. It sits mostly dormant until puberty, and when it wakes up, it isn't really concerned with you at all.

Marcus Hayes

Right, and the Crash Course lesson on this, the one hosted by Hank Green, puts it well. The system is concerned with your alleles, your genetic code, and the future of the human species. Which is a little humbling. Your body spends decades building a machine whose most important job is to hand something off.

Claire Brooks

So today we're covering Chapter 26, the reproductive system, plus the female physiology from that Crash Course episode. And the first thing to get straight is vocabulary. Anatomists split this system into primary and secondary sex organs.

Marcus Hayes

Primary means the gonads. Testes in males, ovaries in females. They do two jobs: they make haploid gametes through meiosis, and they secrete steroid hormones like testosterone, estrogens, and progesterone. Haploid just means half the genetic material, written as n. A normal body cell is diploid, 2n.

Claire Brooks

Secondary sex organs are everything that moves, unites, or nurtures those gametes. In males, that's the spermatic ducts, the accessory glands, and the penis. In females, it's the uterine tubes, the uterus, and the vagina.

Marcus Hayes

And the sex of the offspring comes down to one coin flip. Every egg carries an X chromosome. A sperm carries either an X or a Y. If an X sperm wins, the zygote is XX, female. If a Y sperm wins, it's XY, male. So the sperm decides, which is the sort of detail people get backwards.

Claire Brooks

Let's start with the male side, because it has one of the best engineering problems in the whole body.

Claire Brooks

Sperm production needs a temperature about 2 to 3 degrees Celsius below core body temperature. So roughly 34 degrees instead of 37. That's why the testes hang outside the body in the scrotum, and three structures work together to hold that temperature steady.

Marcus Hayes

The first is the cremaster muscle. It's derived from the internal oblique, and when you're cold, it contracts and pulls the testes up closer to the body. When you're warm, it relaxes and lets them hang lower. The second is the dartos muscle, which is smooth muscle just under the scrotal skin. It wrinkles the skin, which shrinks the surface area and tightens the pouch against heat loss.

Claire Brooks

And the third one is the clever one. The pampiniform plexus.

Marcus Hayes

It's a network of veins in the spermatic cord that wraps around the testicular artery. Think of it as a heat exchanger. The artery brings in blood at body temperature, but the cooler venous blood coming back out of the scrotum surrounds it and absorbs that heat first. So the blood is cooled by about 1.5 to 2.5 degrees before it ever enters the testis. It's like preheating or precooling water by running two pipes side by side, going opposite directions.

Claire Brooks

Which explains two clinical problems. Cryptorchidism is when one or both testes fail to descend. They form in the fetal abdomen near the kidneys and normally travel down along a connective tissue cord called the gubernaculum, through the inguinal rings, in the final months of gestation. If they stay put, core body warmth halts spermatogenesis, and the result can be sterility if it isn't corrected by about age one. The other is varicocele, where those pampiniform veins dilate and look like a bag of worms, because the venous valves are absent or incompetent. It raises scrotal temperature and impairs fertility. About 10 percent of males have one.

Marcus Hayes

Okay, so what's happening inside the testis? A fibrous capsule called the tunica albuginea divides it into 200 to 300 lobules, and each lobule holds tightly coiled seminiferous tubules. The tubule lining is the germinal epithelium, where germ cells become sperm. Supporting them are nurse cells, also called Sertoli cells, which form a blood testis barrier. That barrier protects the haploid cells from autoimmune attack, because to the immune system, they look foreign.

Claire Brooks

And in the connective tissue between the tubules, the interstitial endocrine cells, the Leydig cells, make testosterone when luteinizing hormone from the anterior pituitary tells them to.

Marcus Hayes

The production line goes like this. A Type A spermatogonium is a stem cell. It divides by mitosis, one daughter stays a Type A, so the supply never runs out, and the other becomes a Type B. Type B grows into a primary spermatocyte, diploid, crosses the barrier, and finishes meiosis one as two secondary spermatocytes. Meiosis two splits each of those, so you end with four haploid spermatids from one primary spermatocyte. Four from one. Hold that number, because the female side is different.

Claire Brooks

Then comes spermiogenesis, where a round spermatid becomes a streamlined sperm. It builds an acrosome cap from the Golgi complex, which carries lysosomal enzymes like hyaluronidase to help it penetrate the egg. It sheds excess cytoplasm. And it builds a tail with a midpiece packed with mitochondria.

Marcus Hayes

Then the journey. Seminiferous tubules, the rete testis, efferent ductules, and the epididymis, where sperm mature and can be stored for 40 to 60 days. Next, the ductus deferens, with a thick muscular wall that pushes sperm along by peristalsis and ends in a widened ampulla. Then the ejaculatory duct, which passes through the prostate, and finally the urethra.

Claire Brooks

Then the glands add the liquid. A typical ejaculate is 2 to 5 milliliters, and sperm are only about 10 percent of it. The seminal vesicles, behind the bladder, contribute 60 percent: a yellowish fluid rich in fructose for energy for the flagella, plus prostaglandins that stimulate female peristalsis. The prostate gives 30 percent, a thin milky fluid with citrate, calcium, and prostate specific antigen, PSA, which liquefies the semen after ejaculation. And the bulbourethral glands, Cowper's glands, release a clear alkaline mucus that neutralizes leftover acidic urine in the urethra and lubricates the glans.

Marcus Hayes

The normal count is 50 to 120 million sperm per milliliter. Below roughly 20 to 25 million per milliliter is considered clinical infertility. And for the erection itself, the penis has three cylinders of erectile tissue. Two corpora cavernosa on top, and one corpus spongiosum underneath, which wraps the urethra and expands into the glans. Deep arteries from the internal pudendal artery fill the cavernous spaces.

Marcus Hayes

Now the female side, and I want to begin with a fact that surprises almost everyone. The uterine tubes, the fallopian tubes, are not directly attached to the ovaries.

Claire Brooks

Which seems like a design flaw. So what happens at ovulation?

Marcus Hayes

The secondary oocyte is expelled from the ovary into the peritoneal fluid of the pelvic cavity. Then the fimbriae, the finger like fringe on the end of the tube's infundibulum, capture it. Ciliated cells lining the tube, which are roughly 10 centimeters long, beat toward the uterus and sweep the egg along, while secretory peg cells provide nutrients. Fertilization normally happens in the ampulla of the tube.

Claire Brooks

That gap also explains why ectopic pregnancy exists. Implantation outside the uterine cavity, mostly in the tube, can be life threatening if it ruptures.

Marcus Hayes

The uterus itself has three layers. The perimetrium is the outer serosa. The myometrium is the bulk, interlaced smooth muscle that responds to oxytocin and prostaglandins during labor and menstruation. And the endometrium is the inner lining, with two parts: the stratum functionalis on top, which is shed every cycle, and the stratum basalis underneath, which regenerates it. Blood comes by uterine arteries to arcuate arteries to straight arteries to spiral arteries, and the spiral arteries are the ones that supply the functional layer.

Claire Brooks

Let me bring in the egg timeline, because it's the opposite of sperm. Primordial germ cells migrate from the embryonic yolk sac to the genital ridge at five to six weeks of gestation and become oogonia. They divide by mitosis until early infancy, then become primary oocytes, arrested in prophase of meiosis one. A female infant is born with about one million. Atresia, programmed cell death of follicles, trims that to around 400,000 by puberty.

Marcus Hayes

Hank Green's line in the Crash Course episode is that females are born with essentially all the early versions of eggs they will ever have. So where a male makes sperm continuously, a female's whole supply is already in place, waiting.

Claire Brooks

And the maturing is slow. Each month, a cohort of 20 to 25 primordial follicles is woken up. They need roughly 300 to 375 days to pass through the primary, secondary, and tertiary stages and become a mature Graafian follicle. Usually only one dominant follicle ovulates. The rest undergo atresia.

Marcus Hayes

So the egg you see released on day 14 was recruited almost a year ago. And remember my four from one for sperm? The egg division is lopsided. Meiosis one gives a large secondary oocyte and a tiny polar body. The oocyte stalls at metaphase two, and it only finishes meiosis two if a sperm penetrates it, casting off a second polar body and forming a diploid zygote. No sperm, no completion.

Claire Brooks

Now the part students find hardest: two cycles running in parallel. The ovarian cycle is about the egg, and the menstrual cycle is about the uterine lining. Let's take the ovarian one first, using a 28 day cycle.

Marcus Hayes

Days 1 to 14 are the follicular phase. The hypothalamus releases GnRH, which tells the anterior pituitary to release FSH and LH. FSH drives growth of the dominant follicle, and the granulosa and theca cells around the egg secrete rising estrogen.

Claire Brooks

Here's the twist. Normally, hormones in the body feed back negatively. But when estrogen gets very high, it flips to positive feedback and triggers a massive LH surge. That surge makes the oocyte finish meiosis one, and enzymes dissolve a spot in the ovarian wall. That's ovulation, around day 14.

Marcus Hayes

Days 15 to 28, the luteal phase. The ruptured follicle collapses and becomes the corpus luteum, which secretes lots of progesterone along with estrogen and inhibin. Those exert negative feedback and shut down FSH and LH. If there's no fertilization, the corpus luteum degenerates into scar tissue, the corpus albicans, around days 24 to 26, and hormone levels plummet.

Claire Brooks

And the uterus is reading all of those hormone levels. Days 1 to 5, the menstrual phase. Falling progesterone and estrogen make the spiral arteries spasm and constrict, so the functional layer loses blood, dies, and sloughs off, around 40 milliliters of blood and serous fluid. Days 6 to 14, the proliferative phase. Rising estrogen from growing follicles stimulates the stratum basalis to rebuild, regrow the endometrial glands, and lengthen the spiral arteries.

Marcus Hayes

Days 15 to 28, the secretory phase. Progesterone from the corpus luteum makes the glands swell and secrete glycogen rich mucus and makes the spiral arteries tortuous. The window for implantation opens about seven days after ovulation. If no embryo arrives, the corpus luteum fades, hormones drop, and the cycle restarts. So the ovary writes the schedule, and the uterus follows it.

Claire Brooks

When this timing misfires, you get the disorders. Dysmenorrhea, painful periods, comes from excessive prostaglandins causing ischemic contractions of the myometrium, and it affects up to 75 percent of females aged 15 to 25. Endometriosis is endometrial tissue growing outside the uterus, in the ovaries, tubes, bladder, or bowel. It responds to the same hormones, bleeds internally, and the scarring can cause infertility. Amenorrhea, the absence of periods, is normal in pregnancy and lactation, but pathologic with very low body fat or hypopituitarism.

Marcus Hayes

On the male side, benign prostatic hyperplasia starts around age 45, and by 70, over 90 percent of men have some degree of it. The enlarged prostate compresses the urethra and can lead to bladder and kidney infections. Prostate cancer is the second most common cancer in men, after lung cancer, and it often arises at the periphery without blocking urine flow early on, so screening matters: a digital rectal exam and the PSA test. Caught early, survival is about 80 percent. For cervical cancer, risk factors include HPV, and the Pap smear catches it.

Claire Brooks

And the system eventually winds down. Menopause, typically between ages 45 and 55, comes from exhaustion of ovarian follicles. Estrogen drops cause hot flashes, bone loss, higher cholesterol, and thinning of the vaginal lining. In men, andropause is more gradual, with testosterone and libido declining from the 50s. Erectile dysfunction affects about 20 percent of men in their 50s and 50 percent in their 80s, and about 90 percent can still ejaculate.

Marcus Hayes

So a system that sat quiet for years, ran on precise timing for decades, and then retires. All of it built, as that Crash Course lesson says, around the future of humanity rather than around you.