Sex Hormone

Sex Hormone, any of several chemical substances that affect the development and functioning of the reproductive system in vertebrates, or animals with a backbone.

The sex hormones are divided into three major groups: gonadotropins, gonadal hormones, and lactogens. Gonadotropins stimulate the gonads, which are sperm- or egg-producing organs. The male gonads are the testes, which produce sperm, and the female gonads are the ovaries, which produce eggs. Gonadotropins are secreted by the pituitary gland, which is located in the center of the brain and is controlled by an area of the brain known as the hypothalamus. Gonadotropins, such as the leutinizing hormone (LH) and the follicle-stimulating hormone (FSH) in females and the interstitial cell-stimulating hormone (ICSH) in males, control the development and functions of the ovaries and testes, including menstruation and sperm production. Gonadal hormones such as estrogen, progesterone, and testosterone are secreted primarily by the testes and ovaries, placenta (the sac of nutritive tissue that supports and protects the fetus), and adrenal glands. Their chief function is to regulate the development of the secondary sex characteristics such as deepening of the voice in males and distribution of body hair (see also Human Sexuality). The third group, the lactogens, are secreted by the pituitary gland and are necessary for the secretion of milk in the mammary glands of mammals. They are also believed to affect maternal behavior patterns.

Little is known about the presence of sex hormones in invertebrates (animals without backbones). However, experiments have shown that some of these animals, including earthworms and various crustaceans, have chemical substances whose functions are similar to those of the sex hormones in vertebrates.