ESTROGEN

any of a group of hormones that primarily influence the female reproductive tract in its development, maturation, and function. There are three major hormonesestradiol, estrone, and estriolamong the estrogens, estradiol being the predominant one. The major sources of estrogens are the ovaries and the placenta (the temporary organ that serves to nourish the fetus and remove its wastes); additional small amounts are secreted by the adrenal glands and by the male testes. It is believed that the egg follicle (the saclike structure that holds the immature egg) and interstitial cells (certain cells in the framework of connective tissue) in the ovary are the actual production sites of estrogens in the female. Estrogen levels in the bloodstream seem to be highest during the egg-releasing period (ovulation) and after menstruation, when tissue called the corpus luteum replaces the empty egg follicle. Estrogens affect the ovaries, vagina, fallopian tubes, uterus, and mammary glands. In the ovaries, estrogens help to stimulate the growth of the egg follicle; they also stimulate the pituitary gland in the brain to release hormones that assist in follicular development. Once the egg is released, it travels through the fallopian tubes on its way to the uterus; in the fallopian tubes estrogens are responsible for developing a thick muscular wall and for the contractions that transport the egg and sperm cells. The young female uterus, if deprived of estrogens, does not develop into its adult form; the adult uterus that does not receive estrogens begins to show tissue degeneration. Estrogens essentially build and maintain the endometriuma mucous membrane that lines the uterus; they increase the endometrium's size and weight, cell number, cell types, blood flow, protein content, and enzyme activity. Estrogens also stimulate the muscles in the uterus to develop and contract; contractions are important in helping the wall to slough off dead tissue during menstruation and during the delivery of a child and of the placenta. The cervix, the tip of the uterus, which projects into the vagina, secretes mucus that enhances sperm transport; estrogens are thought to regulate the flow and thickness of the mucous secretions. The growth of the vagina to its adult size, the thickening of the vaginal wall, and the increase in vaginal acidity that reduces bacterial infections are also correlated to estrogen activities. In the breasts the actions of estrogens are complexly interrelated with those of other hormones, and their total significance is not easily defined; they are, however, responsible for growth of the breasts during adolescence, pigmentation of the nipples, and the eventual cessation of the flow of milk. Estrogens also influence the structural differences between the male and female bodies. Usually the female bones are smaller and shorter, the pelvis is broader, and the shoulders are narrower. The female body is more curved and contoured because of fatty tissue that covers the muscles, breasts, buttocks, hips, and thighs. The body hair is finer and less pronounced, and the scalp hair is usually more permanent. The voice box is smaller and the vocal cords shorter, giving a higher-pitched voice in females than in males. In addition, estrogens suppress the activity of sebaceous (oil-producing) glands and thereby reduce the likelihood of acne in the female. In experimental animals, loss of estrogens diminishes the mating desires and other sexual behaviour patterns. In the male, traces of estrogens are present in the blood and urine; estrogens seem to be most evident in the male during puberty and old age. Their function in the male and their interplay with the male hormones are not completely known.