Reproduction

The testes are the paired gonads, and they develop inside the abdomen but come to lie before birth in the scrotum, a pouch of skin outside the abdomen between the thighs. The reason is temperature. Spermatogenesis needs a site about $2^\circ\text{C}$ below core body temperature, which puts the working figure at about $35^\circ\text{C}$ against a body temperature of $37^\circ\text{C}$. The scrotum, not the testes, is the structure that maintains that temperature.

Inside, each testis is built of lobules, and each lobule holds one to four tightly coiled seminiferous tubules, which is where sperm are made. The route out is worth learning as an order rather than as a list: seminiferous tubules to the rete testis, out through the efferent ductules to the epididymis, which is coiled on the outer surface of the testis and is where sperm mature and are stored, then into the vas deferens, which joins the duct of the seminal vesicle to form the short ejaculatory duct, enters the prostate and empties into the urethra. That urethra carries urine as well as sperm, which is why it keys as a urogenital organ rather than a purely reproductive duct.

The accessory glands supply the liquid. The paired seminal vesicles sit where the sperm duct meets the ejaculatory duct and contribute about $60$ of the total volume of semen; the prostate encircles the urethra just below the bladder, is the largest of the semen-producing glands and empties directly into the urethra; the bulbourethral glands, also called Cowper's glands, lie at the junction of the ejaculatory duct and the urethra and secrete a clear mucus before ejaculation that neutralises acidic urine still in the urethra, which is exactly what the papers key. The finished product is $2$ to $5\ \text{ml}$ of semen carrying between $20$ and $150$ million sperm per $\text{ml}$, and the prostaglandins in it help transport by stimulating contractions of the uterus.

The hormonal axis is short. At the onset of puberty the hypothalamus releases gonadotropin-releasing hormone, which makes the anterior pituitary release FSH and LH. FSH enters the testis and acts on the Sertoli cells, which facilitate spermatogenesis and secrete a fluid giving developing sperm a liquid medium, protection and nourishment; Sertoli cells also secrete inhibin. LH, called interstitial cell-stimulating hormone, or ICSH, in the male, acts on the interstitial cells, the Leydig cells, to make testosterone, and testosterone drives the secondary sexual characteristics, the deepening of the voice and the growth of facial, axillary and pubic hair. Both loops close negatively: rising testosterone inhibits GnRH, FSH and LH, and inhibin, released when the sperm count is too high, inhibits GnRH and FSH, so that when the count reaches $20$ million per $\text{ml}$ the Sertoli cells stop releasing it and the count can climb again.