In the body of a future girl, egg precursor cells are formed even during her intrauterine development. In the ovary of a newborn girl there are about 40 thousand immature germ cells, of which only 350–500 reach full maturity. Their maturation occurs in the graaf vesicles (follicles) of the ovaries.
In a mature organism, every month (every 26–32 days) in one of the ovaries, under the influence of a specific hormone from the pituitary gland, one follicle, containing the future egg, matures. The process of release of an egg from a follicle is called ovulation.
An immature egg enters the abdominal cavity, from where it enters the oviduct (fallopian tube) through a fringed funnel and completes its maturation there, moving for 7 days along the fallopian tube to the uterus.
The oocyte moves to the uterus due to the contractions of smooth muscles located in the walls of the oviduct, and the movements of the cilia of the epithelium covering its inner surface. In the oviduct, the maturation of the egg is completed and fertilization can occur. If it happens, then pregnancy occurs. If fertilization does not occur, then the egg dies.
The cavity of the ruptured follicle is gradually filled with cells containing a fat-like substance of yellow color, and turns into a corpus luteum – a temporary endocrine gland that produces the hormone progesterone.
The corpus luteum hormone delays the maturation of the next follicle and prepares the lining of the uterus to accept the embryo.
If the egg is fertilized, then progesterone will keep the pregnancy going. If pregnancy has not occurred, progesterone ceases to be released on the 13-14th day after ovulation, and the uterine lining is rejected. In this case, rather large blood vessels burst. Pieces of mucous membrane along with blood flow into the vagina. This period – menstruation – lasts from 3 to 5 days. Then the lining of the uterus is restored.
Ovulation and, accordingly, the menstrual cycle (maturation of the follicle, ovulation, formation and reverse development of the corpus luteum), is repeated every 26–32 days.
A woman is capable of reproduction for about 30 years. Then the ovaries stop producing eggs.