Estrogens (alternate spellings: oestrogens or œstrogens) are a group of steroid compounds, named for their importance in the estrous cycle, and functioning as the primary female sex hormone.
Estrogens are used as part of some oral contraceptives, in estrogen replacement therapy of postmenopausal women, and in hormone therapy for transsexual women.
Like all steroid hormones, estrogens readily diffuse across the cell membrane; inside the cell, they interact with estrogen receptors.
Types of estrogen
The three major naturally occurring estrogens in women are estradiol, estriol, and estrone. In the body these are all produced from androgens through actions of enzymes.
- From menarche to menopause the primary estrogen is 17β-estradiol. In postmenopausal women more estrone is present than estradiol.
- Estradiol is produced from testosterone and estrone from androstenedione.
- Estrone is weaker than estradiol.
A range of synthetic and natural substances have been identified that also possess estrogenic activity. Synthetic substances of this kind are known as xenoestrogens, while natural plant products with estrogenic activity are called phytoestrogens.
Estrogen is produced primarily by developing follicles in the ovaries, the corpus luteum, and the placenta. Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) stimulate the production of estrogen in the ovaries. Some estrogens are also produced in smaller amounts by other tissues such as the liver, adrenal glands, and the breasts. These secondary sources of estrogen are especially important in postmenopausal women.
Synthesis of estrogens starts in theca interna cells in the ovary, by the synthesis of androstenedione from cholesterol. 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. The conversion of testosterone to estradiol, and of androstenedione to estrone, is catalyzed by the enzyme aromatase.
While estrogens are present in both men and women, they are usually present at significantly higher levels in women of reproductive age. They promote the development of female secondary sex characteristics, such as breasts, and are also involved in the thickening of the endometrium and other aspects of regulating the menstrual cycle. In males estrogen regulates certain functions of the reproductive system important to the maturation of sperm.   and may be necessary for a healthy libido .
Estradiol levels vary through the menstrual cycle, with levels highest just before ovulation.
- promote formation of female secondary sex characteristics
- stimulate endometrial growth
- increase uterine growth
- maintenance of vessel and skin
- reduce bone resorption, increase bone formation
- increase hepatic production of binding proteins
- increase circulating level of factors 2,7,9,10, antithrombin III, plasminogen
- increase platelet adhesiveness
- Fluid balance
- salt and water retention
- gastrointestinal tract
- reduce bowel motility
- increase cholesterol in bile
- About 80% of breast cancers, once established, rely on supplies of the hormone estrogen to grow: they are known as hormone-sensitive or hormone-receptor-positive cancers. Suppression of production in the body of estrogen is a treatment for these cancers.
Studies have found better correlation between sexual desire and androgen levels than for estrogen levels.
In studies involving mice and rats, it was found that lung function may be improved by estrogen. In one study involving 16 animals, female mice that had their ovaries removed to deprive them of estrogen lost 45 percent of their working alveoli from their lungs. Upon receiving estrogen, the mice recovered full lung function.
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