The Roles of Estrogen in the Female Reproductive System
Estrogens are a group of hormones that play an important role in the formation of female secondary sexual characteristics, like pubic and armpit hair and breasts, and are responsible for the regulation of the reproductive system and menstrual cycle. Estrogen levels rise during puberty, leading to secondary sex characteristics, like breasts and changes in overall body composition. The ovaries start estrogen production, triggering the monthly menstrual cycle. During a woman’s menstrual cycle, estrogen creates an environment suited for fertilization, implantation, and nurturing of an embryo. Estrogen stimulates the growth and release of the egg. The fallopian tubes have thick walls and contractions needed to transport the egg to the uterus thanks to estrogen. In the uterus, estrogen enhances and maintains the uterine lining, enhances blood flow, proteins, and enzymes, stimulates the uterine muscles to contract during childbirth, and assists in removal of dead tissue during menstruation. Estrogen regulates uterine mucous secretion thickness and flow to enhance sperm movement. In the vagina, estrogen helps develop the vagina into its adult size, thickens the vaginal wall, increases vaginal acidity to reduce bacterial infections, and helps with lubrication. Estrogen also is responsible for growth and nipple pigmentation during puberty, and it stops the flow of milk once breastfeeding ends. Estrogens also regulate important processes in a woman's skeletal, cardiovascular, and central nervous systems.
The Three Kinds of Estrogen
The body has two main receptors to which estrogen binds: alpha receptors and beta receptors. Each of the three type of estrogens (described below) bind with varying affinity to the receptors.
This is the major hormone produced in reproductive age women. It is made primarily by the ovaries but also the adrenal glands, fat, liver, breasts, and brain. Estradiol also is made from the breakdown of testosterone. In addition to its reproductive system functions, estradiol also regulates:
Bone formation and turnover
Estrone is the dominant form of estrogen produced by a menopausal woman. Estrone is primarily formed in the adrenal glands and fatty tissue, with some produced by the ovaries. As the ovaries age, estradiol production decreases, and estrone concentration in the body rises to make up for the loss. The increase in belly fat in menopausal women further increases the estrone production. Estrone is less potent at estrogen receptors than estradiol so even if a woman's total estrogen remains the same, when the ratio tilts toward estrone instead of estradiol, the body may register a practical decrease in estrogen. High levels of estrone may result in systemic inflammation, which can lead to sore joints, impaired gut performance, and fluid retention and puffiness.
Estriol concentrations are highest during pregnancy, when it is secreted by the placenta. In non-pregnant women, estriol is barely detectable. Estriol is not synthesized in or secreted from the ovaries but instead is produced primarily as a breakdown product of estradiol and estrone. Estriol is the weakest of the estrogens.
The most common estrogen used in menopause hormone therapy (MHT) is estradiol. The notable exception is conjugated equine estrogens, which according to the manufacturer of the drug, contains 10 different estrogens and estrogen metabolites.
Given the large number of roles played by estrogens in the body, it's no wonder that radical changes, in puberty and again at menopause, cause disruptions in the lives of girls and women experiencing them. Since all women will experience these changes, it is critical that more research be funded to find ways to reduce symptoms and alleviate suffering.
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