The human reproductive systems and gametes

Humans are mammals. The embryo grows in the uterus until birth, the eggs are small with little yolk, few young are born, and the young need years of care. The male reproductive system has two testes, the epididymis, the vas deferens, accessory glands and the urethra. It makes sperm and male hormones. At puberty the hormones bring the secondary sex features: a deeper voice, stronger muscles and facial hair. The testes lie outside the body cavity in the scrotum, which is cooler than the body, and this suits the making of sperm. A testis that does not descend into the scrotum does not make sperm, and a doctor is the person to advise on that. The testis has two jobs: it makes sperm, and it makes the hormone testosterone. Each testis leads to a coiled tube, the epididymis, which leads to the vas deferens. The vas deferens carries sperm to the urethra. The seminal vesicles make an alkaline fluid with fructose sugar that feeds the sperm. The prostate gland and Cowper’s glands make a alkaline fluid that neutralizes the acidity of the urethra just before the sperm pass, because a neutral medium suits them. The penis is an organ of spongy tissue that holds the urethra, and urine and sperm leave it separately. A section of a testis shows the seminiferous tubules, with interstitial cells between them that make testosterone. Inside each tubule the Sertoli cells nourish the sperm, and may also have a protective role. The tubules are lined with primary germ cells (2N), which divide and finally become sperm.