Estrogens (also called Oestrogens) are steroid compounds that are important for development and functioning of females of the species. They are named so because they play an important role in the estrous cycle. Their name comes from estrus/oistros (period of fertility for female mammals) + gen/gonos = to generate.
Estrogens are used as a part of some oral contraceptives, hormone replacement therapies and other treatment for disorders on the endocrine system.
Secretion of estrogens
Estrogens are produced primarily by the ovaries. They are released by the follicles on the ovaries and are also secreted by the corpus luteum after the egg has been released from the follicle and from the placenta.
The stimulation for secretion of estrogen comes from the Luteinizing hormone (LH) from anterior pituitary gland. The estrogen is synthesized in the theca interna cells in the ovary.
The primary molecule that begins the synthesis of estrogen is cholesterol. It forms androstenodione. Androstenedione is a substance of moderate androgenic activity. This compound crosses the basal membrane into the surrounding granulosa cells, where it is converted to estrone or estradiol, either immediately or through testosterone.
This conversion of testosterone to estradiol, and of androstenedione to estrone, is catalyzed by the enzyme aromatase. Estradiol levels vary through the menstrual cycle, with levels highest just before ovulation
Secondary sources of estrogen
Some of the estrogens are also produced in smaller amounts by other tissues such as the liver, adrenal glands, and the breasts. These secondary sources are important in women who have already had their menopause.
Fat cells are also sources of estrogen. This is the reason why underweight or overweight women are at risk of infertility. Delicate balance of estrogen is important for fertility. Excess or deficiency of this hormone affects fertility adversely.
Mechanism of action
Estrogens diffuse across the cell membrane. Once inside they bind to the estrogen receptors present in the nucleus of the cells. They also act by activating G protein-coupled receptor, GPR30. Their main action is by regulating the expression of certain genes that maintain fertility and help in development of the female reproductive system.
It was nearly 3 decades ago when Jensen and Jacobsen found that estrogen acts via a specific receptor protein. In 1986 this receptor protein was cloned and the term Estrogen Receptor or ER was coined.
Until 1995, it was assumed that there was only one ER and that it was responsible for mediating all of the physiological and pharmacological effects of natural and synthetic estrogens and its antagonists. In 1995 a second ER, called ER beta was found in the male reproductive tissues. The first ER is now called ER alpha.