A 30-centimetre duct, three glands and a temperature-controlled pouch
The male reproductive system is a set of organs outside the body and inside the pelvis that makes sperm and semen. The testes produce sperm in coiled tubules and hormones in cells between them, while muscles in the scrotum raise or lower the testes as the temperature changes.
The main external organs are the penis and the scrotum, the skin pouch that holds the testicles. Inside the penis runs the urethra, which carries both semen and urine out through a single opening. During arousal, spaces in its erectile tissue, the corpora cavernosa and corpus spongiosum, fill with blood as the arteries widen and the veins are squeezed shut; the pudendal artery supplies it.
When it is cold the cremaster muscle draws the scrotum towards the body and the dartos layer wrinkles it; in warmth both relax and the scrotum hangs lower and smoother. A spermatic cord of artery, vein and nerve links each testis to the abdomen through the inguinal canal. The testes make sperm by meiosis in the seminiferous tubules, while Leydig cells between the tubules make androgens. Sperm then pass into the epididymis, a tightly coiled whitish tube where ion channels help them mature and concentrate, and on through the vas deferens, a duct about 30 centimetres long, to the ejaculatory duct. Seminal vesicles, the prostate and the bulbourethral glands add the fluids that nourish sperm and lubricate the ducts.
A male develops when a sperm carrying a Y chromosome fertilises the egg. Testosterone from the embryonic testes shapes the reproductive organs, though inside target cells an enzyme called 5α-reductase first converts it into dihydrotestosterone, the form that acts. At six weeks the external genitalia of the two sexes still look the same; by 10 to 12 weeks they are distinct, and they are fully formed by week 16.
Before birth the gubernaculum, a cord attached to each testis, guides it down into the scrotum, and if the passage fails to close behind it an inguinal hernia or hydrocele can result. Male germ cells also carry special DNA repair systems that protect the genome passed to offspring.
Source: Male reproductive system
