Estrogens

ESTROGENS

(Female Sex Hormones)

These are substances which can induce estrus in spayed animals.

It was established in the year 1900 that ovaries control female reproductive function through a hormonal mechanism. Allen and Doisy (1923) found that an alcoholic extract of ovaries was capable of producing estrus and devised a simple bioassay method. The active principle was obtained in pure form in 1929 and soon its chemical structure was worked out.

Natural estrogens Estradiol is the major estrogen secreted by the ovary. It is synthesized in the graafian follicle, corpus luteum and placenta from cholesterol. Steps depicted on the right hand side in Fig. 20.1 are carried out. Further steps are shown below.

Estradiol is rapidly oxidized in liver to estrone which is hydroxylated to form estriol. All three are found in blood, but estradiol is the most potent estrogen. Small quantity (2–20 μg/day) of estradiol is derived in human males also from aromatization of testosterone in the testes and extraglandular tissues. In mare, large quantity of equilin is produced which has 1/5 estrogenic potency of estradiol.

Synthetic estrogens Natural estrogens are inactive orally and have a short duration of action due to rapid metabolism in liver. To overcome this, synthetic compounds have been produced:

Steroidal     Ethinylestradiol, Mestranol, Tibolone.

Nonsteroidal        Diethylstilbestrol (stilbestrol), Hexestrol, Dienestrol

The nonsteroidal compounds assume a trans configuration as depicted below and sterically resemble natural estrogens.

Regulation Of Secretion

The daily secretion of estrogens in menstruating women varies from 10– 100 μg depending on the phase of the cycle. Secretion starts from the graafian follicle under the influence of FSH and the blood level rises gradually during the follicular phase. Due to the modest preovulatory FSH surge, estrogens further rise transiently. After ovulation, corpus luteum continues to secrete estrogens till about two days before menstruation. Estrogens exercise feedback inhibition of FSH (also LH at higher concentrations) by direct action on pituitary as well as through hypothalamus.

During pregnancy, placenta secretes large quantities of estrogens, reaching a peak of upto 30 mg/day at term. Their level declines sharply after delivery. In the postmenopausal women, daily production of estrogen has been estimated as 2–10 μg—derived primarily by extraglandular aromatization of adrenal androgens.