Question

During the menstrual phase, What happened to the endometrial lining? Are blood levels of estrogen and progesterone low or high?

Answer 1

Menstrual cycle is the changes occurring the innermost uterine lining in response to ovarian hormones estrogen and progesterone. Endometrium is innermost lining of the uterus to where the blastocyst attaches during implantation. This layer has luminal epithelium, glandular epithelium and stroma. The menstrual cycle has the menstrual phase, follicular phase and the secretory phase. In follicular phase, the endometrial cells proliferate in response to estrogen. In secretory phase, progesterone secreted by corpus luteum causes differentiation of the endometrial cells. However, when the estrogen and progesterone levels decline, due to decreases ovarian activity, the menstrual phase set in.

Progesterone levels decline due to regression of the corpus luteum at end of secretory phase. Estrogen levels decline as the FSH levels are low. Hence, granulosa cell secrete minimal estrogen. Menstrual phases usually last between 1-5 days. The loss of ovarian hormones causes the endometrial cells to undergo apoptosis. Withdrawal of progesterone causes the spiral arterioles to constrict and increasingly coil. The decreased ischemia causes reduced flow of blood to the endometrium. As a result, endometrial tissue degrades. There are increased contractions of the smooth muscles in uterus due to prostaglandin produced. Thus, in menstrual phase, there is loss of endometrial tissues. This loss of endometrial tissues also exposes the spiral arterioles. The increased contractions lead to bleeding from the spiral arterioles which will flush the degraded endometrial tissues. After 2 days of bleeding, the FSH will induce estrogen production by granulosa cells of the ovarian follicle. This estrogen will induce mitosis in the endometrial cell leading to their proliferation. These cells will then cover the spiral arterioles, leading to decreased menstruation. Estrogen also causes prolonged vasoconstriction. This leads to formation of a clot over the denuded blood vessels. Thus, finally the menstrual phase will end and the proliferative phase is induced. In menstrual phase, estrogen and progesterone levels are low in circulation.