Transcript
Estrogen, in females, is produced primarily by the ovaries, and during pregnancy, the placenta. Estrogen functions mainly to promote the development of female secondary sexual characteristics, such as breasts, and is also involved in the thickening of the endometrium and other aspects of regulating the menstrual cycle. Like most sex hormones, estrogens and androgens, are fat soluble and water repellent. In other words, they "like" lipid or fatty structures such as those surrounding cells but are generally repelled by watery areas. Steroids generally travel to their target cells attached to a special carrier protein that "likes" water . On the other hand, Most water-soluble hormones, like the amino acid derivatives and peptides, can travel freely in the blood because they "like" water. However, they are repelled by lipid or fatty structures such as the membranes that surround the cell and nucleus. Because of this, these hormones generally bind to receptor sites on the outside of the cell and signal from there.
Follicle-stimulating hormones (FSH) stimulates the ovarian production of estrogens by the granulosa cells of the ovarian follicles and corpora lutea. Some estrogens are also produced in smaller amounts by other tissues such as the liver, adrenal glands, and the breasts. Estrogen is initiated in the simple endocrine pathway, and synthesizes in theca interna cells in the ovary, by the synthesis of androstenedione from cholesterol. Androstenedione is then converted into androgens such as testosterone as well as estrogen. This compound crosses the basal membrane into the surrounding granulosa cells, where it is converted either immediately into estrone, or into testosterone and then estradiol in an additional step. The conversion of androstenedione and testosterone into estrone and estradiol, is catalyzed by aromatase, enzymes which are expressed in granulosa cells. The actions of estrogen are controlled by the estrogen receptor, a nuclear protein that binds to DNA and controls gene expression. Estrogen enters passively into the cell where it binds to, and activates the estrogen receptor. Since estrogen enters all cells, its actions are dependent on the presence of the Estrogen Receptor in the cell. The Estrogen Receptor is expressed in specific tissues including the ovary, uterus and breast. Estrogen regulation is also known as a negative feedback loop, because it inhibits the production of certain hormones. The process starts when the hypothalamus notices a low level of estradiol in the blood and begins releasing a hormone known as gonadotropin releasing hormone (GnRH), which notifies the pituitary gland to make and release both the luteinizing hormone (LH) and the follicle-stimulating hormone (FSH). In females, LH and FSH tell the ovaries to secrete estradiol and progesterone, which stimulates the growth of the egg-producing ovarian follicle and prepares the uterus for pregnancy. In males, LH and FSH notify the testes to secrete testosterone, which stimulates sperm production. The ovaries make and release estradiol until a certain level is reached in the bloodstream. The hypothalamus and pituitary notice the increase and stop secreting GnRH, LH and FSH hormones. This causes the ovaries to stop releasing estradiol and progesterone until the level in the blood decreases to a low enough level that then reactivates the process.
Citations
"E.Endocrine System : Types of Hormones." E.hormone. Tulane University, 2014. Web. 03 Mar. 2014.
"Endocrine System : Feedback Loops." E.hormone. Tulane University, 2014. Web. 03 Mar. 2014.
"Estrogen." Wikipedia. Wikimedia Foundation, 20 Feb. 2014. Web. 01 Mar. 2014
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