Testis and ovary

A pair of testes lies in the scrotal sac, outside the abdomen. The testis has two jobs: it is the primary male sex organ and an endocrine gland.

The testis is made of seminiferous tubules and stromal (interstitial) tissue. The Leydig or interstitial cells, in the spaces between the tubules, produce androgens, mainly testosterone.

Androgens control how the male accessory sex organs (epididymis, vas deferens, seminal vesicles, prostate gland, urethra and others) develop, mature and work. They also build muscle, bring on facial and axillary hair, and give aggressiveness and a low-pitched voice.

Androgens have a major stimulatory role in spermatogenesis. They act on the central neural system to influence male sexual behaviour (libido), and they have anabolic (synthetic) effects on protein and carbohydrate metabolism.

A pair of ovaries lies in the female abdomen. The ovary is the primary female sex organ, producing one ovum each menstrual cycle, and it also makes two groups of steroid hormones: estrogen and progesterone.

The ovary is made of ovarian follicles and stromal tissue. Estrogen is made mainly by the growing follicles. After ovulation the ruptured follicle becomes the corpus luteum, which secretes mainly progesterone.

Estrogens stimulate the growth and activity of the female secondary sex organs, the development of growing ovarian follicles, the appearance of female secondary sex characters (such as a high-pitched voice) and mammary gland development. They also regulate female sexual behaviour.

Progesterone supports pregnancy. It also acts on the mammary glands, stimulating the formation of alveoli, the sac-like structures that store milk, and milk secretion.