Biology 4.1 - The Menstrual Cycle

Follicle Stimulating hormone (FSH) is secreted by the Anterior Pituitary Gland and causes a primary follicle to mature. It develops a fibrous layer, called the theca, in the developing secondary follicle. These thecal cells secrete oestrogen. 

Theca = outermost layer. 

Following menstruation, the increasing levels of oestrogen in the blood help to rebuild the endometrium that has recently shed. 

Oestrogen also inhibits the production of FSH so that no more follicles can mature. 

Oestrogen stimulates the production of Luteinising Hormone (LH).

LH is secreted by the anterior pituitary gland (around day 14). It initiates OVULATION: the Graafian follicle at the surface of the ovary bursts and releases a seconday oocyte. The secondary oocyte is picked up by fimbriae (singlular: fimbria) and begins its journey towards the uterus.

The Graafian follicle becomes the corpus luteum ('yellow body'). This secretes progesterone which maintains the endometrium in preparation for pregnancy. Progesterone also inhibits the production of both LH and FSH so that no more follicles mature.

IF IMPLANTATION DOES NOT OCCUR, falling FSH and LH levels cause the corpus luteum to degenerate, and so progesterone production declines. 

The falling progesterone levels cause the endometrium to shed. Oestrogen levels are now low, as it is no longer required to rebuild the endometrium. LH and FSH production are no longer inhibited, and so the cycle begins again. 

SUMMARY

FSH = Secreted by the anterior pituitary gland, follicle development.

LH = Secreted by the anterior pituitary gland, ovulation.

Oestrogen = Secreted by the thecal cells of the secondary follicle, rebuild endometrium.

Progesterone = Secreted by the corpus luteum, maintains endometrium in preparation for pregnancy.

• Both pituitary hormones.

• Oestrogen and progesterone = ovarian hormones.