Ho

FIGURE 7 Biosynthesis of estrogens during pregnancy. Note that androgens formed in either the fetal or maternal adrenals are the precursors for all three estrogens and that the placenta cannot convert progesterone to androgens. Hydroxylation of dehydroepiandrosterone sulfate on carbon 16 by the fetal liver gives rise to estriol, which is derived almost exclusively from fetal sources. Fetal androgens are secreted as sulfate esters and must first be converted to free androgens by the abundant placental sulfatase before conversion to estrogens by the enzyme P450aromatase (P450arom). 3^HSD, 3^-hydroxysteroid dehydrogenase.

and secretory activity of the fetal adrenals are controlled by ACTH, the actions of which are augmented by a variety of fetal growth factors including insulin-like growth factor II (IGF-II). The fetal pituitary is the main source of ACTH, but the placenta also secretes some ACTH. In addition, the placenta secretes corticotropin-releasing hormone (CRH), which not only stimulates the fetal pituitary to secrete ACTH but also directly stimulates steroidogenesis by the fetal adrenal glands.

The chief product of the fetal zone is the 19-carbon androgen dehydroepiandrosterone (DHEA), which is secreted as the biologically inert sulfate ester (DHEA-S). Sulfation protects against masculinization of the genitalia in female fetuses and prevents aromatization in extra-gonadal fetal tissues. The fetal zone produces DHEA-S at an increasing rate that becomes detectable by about the eighth week of pregnancy, well before cortisol and aldosterone are produced by the definitive and transitional zones. At term, secretion of DHEA-S may reach 200 mg per day. The cholesterol substrate for DHEA-S production is synthesized in both the fetal liver and fetal adrenals. It is likely that pregnenolone released into the fetal circulation by the placenta also provides substrate.

Much of the DHEA-S in the fetal circulation is oxidized at carbon-16 in the fetal liver to form 16a-DHEA-S

and then exported to the placenta. The placenta is highly efficient at extracting 19 carbon steroids from both maternal and fetal blood. It is rich in sulfatase and converts 16a-DHEA-S to 16a-DHEA prior to aromati-zation to form estriol. Because synthesis of estriol reflects the combined activities of the fetal adrenals, the fetal liver, and the placenta, its rate of production, as reflected in maternal estriol concentrations, has been used as an indicator of fetal well-being. DHEA-S that escapes 16a-hydroxylation in the fetal liver is converted to androstenedione or testosterone in the placenta after hydrolysis of the sulfate bond and then aromatized to form estrone and estradiol.

Get Rid of Gallstones Naturally

Get Rid of Gallstones Naturally

One of the main home remedies that you need to follow to prevent gallstones is a healthy lifestyle. You need to maintain a healthy body weight to prevent gallstones. The following are the best home remedies that will help you to treat and prevent gallstones.

Get My Free Ebook


Post a comment