FEMALE REPRODUCTIVE SYSTEM
During embryonic life, a stock of cells called oocytes is formed in the womb from puberty until menopause.
A new oocyte is expelled each month by one of the two ovaries.
It is collected by the atrium of the fallopian tube during ovulation.
It is therefore a cyclical phenomenon, but it represents only one of the stages of ovarian function.
At the beginning of a cycle, some oocytes are preparing to be expelled; they are contained within follicles. The size of the follicle increases during the first 14 days of the cycle.
One of them becomes more imposing than the others; it is the one that will be released on the fourteenth day.
After ovulation, what remains of the follicle in the ovary stays for 14 days, then degenerates and the cycle begins again.
During the ovarian cycle, another organ changes its appearance - it is the uterus.
During the first 5 days of the cycle, the uterine lining is expelled in the form of blood. These are menstruations; this mucous membrane is reconstituted during the 9 days after the period, during which its size increases.
After ovulation and during the last 14 days of the cycle, the mucous membrane changes in appearance. It forms the vascularized uterine endothelium, and these transformations make it suitable to receive an embryo. In cases where unprotected sex occurs, it results in fertilization between the egg and a sperm.
The properties of the cervix are also synchronized with the ovarian cycle. During the first 11 days of the cycle, the mucus present in the cervix forms a tight mesh that leaves little chance for sperm to penetrate the uterine cavity during sexual intercourse. Although some still manage to get there.
Within a few days of ovulation, from approximately day 12 to day 16, this mesh relaxes. We say that the cervical mucus becomes sticky, allowing sperm to pass through much more easily. This is the time when fertilization is most likely to occur and result in late pregnancy.
The cycle's mesh tightens, and the uterine cavity becomes less accessible again.
The synchronicity of the ovarian and uterine cycles is not accidental. During the cycle, the ovary secretes two hormones in greater or lesser quantities to act on the uterus: estrogen and progesterone.
At the beginning of the cycle, the blood concentration of both hormones is very low. This is what causes menstruation.
Estrogen is secreted during the first nine days.
The uterus then reacts by regenerating its mucous membranes. Before the 11th day, the limited concentration of estrogen acts on the cervix, inducing a tight mesh. After the 11th day, the estrogen concentration reaches a threshold that involves the relaxation of the mesh, which becomes fibrous. After ovulation, on the 14th day, progesterone is secreted especially. This hormone induces changes in the mucosa and the return to a tight mesh of cervical mucus.