Female gametes (eggs) are produced in the ovaries cyclically, every 28 days. This is a complex process regulated by the hormones LH and FSH which are secreted by the hypophysis.
At the embryonic stage and before the woman is born, the female germ cells called oogonia move towards where the ovaries will be placed. There are about two million of these cells (called oocytes or eggs) in the ovaries of a new-born girl. However, this number decreases quickly so that a young woman has only 800.000 eggs of which only approximately 400 will ovulate during her fertile life.
Hormones such as Estradiol, LH and FSH cause each primary germ cell to undergo a modification of its genetic material. This process is called meiosis, and its aim is to reduce the number of chromosomes to 23 (the other cells have 46). After the first meiotic division, from 46 to 23 chromosomes, the meiosis stops. At this stage the eggs are not capable of being fertilised.
On the first day of each menstrual cycle several follicles, each of which contains an egg, begin to develop simultaneously under the effect of the FSH hormone secreted by the hypophysis gland situated at the base of the brain. Under normal conditions only one of them matures, ovulates, and can be fertilised. The rest disintegrate.
During this process the developing follicles segregate the Estradiol hormone which acts on the hypophysis and induces it to produce the LH hormone. At the same time, the Estradiol induces the endometrial growth in the uterus. On the twelfth day of the cycle the secreted LH stimulates the first meiotic division of the chosen egg. Moreover, the presence of the FSH hormone decreases. Between 38 and 40 hours later, the follicle releases the egg which is picked up by one of the Fallopian tubes. This process is called ovulation and takes place during the fourteenth day of the cycle.
The released egg will survive for approximately one day, and can only be fertilised during this time. It is placed in the ampulla fragment of the Fallopian tube surrounded by the spermatozoa which arrive there, attracted to it. When one spermatozoon enters its cytoplasm the egg immediately becomes impermeable and the others cannot do the same. The tail of this spermatozoon stays outside and its head causes the reactivation of the second meiotic division in the egg.
Twenty-four hours after the spermatozoon enters the egg, the embryo (called the preembryo) has two pronuclei and 46 chromosomes, like all the other cells of the organism. From this moment on the preembryo begins the process of continuous mitotic division which leads to a two-cell embryo, then four-cell, eight-cell and so on.
Meanwhile, the empty follicle (known as the corpus luteum, or yellow body) acquires the properties of the endocrine gland and secretes progesterone and a certain quantity of Estradiol. This induces the preparation of the endometrium (which has already proliferated thanks to the Estradiol) so that it can receive the embryo. The progesterone, which also affects the hypophysis in an inhibitive way, brings about the end of the secretion of LH and, if the pregnancy does not occur, the luteus body degenerates fourteen days after being formed and the endometrium leaves the body with the menstrual blood (28 days after the beginning of the cycle).
The preembryo develops in the fallopian tube for four or five days. At the 12 to 24-cell stage this preembryo, now called morula, enters the uterus where it implants in the endometrium. This usually occurs on the sixth day, at blastocyst stage. At this point it begins to secrete the hCG hormone which has a similar structure to the LH. For this reason the luteus body does not degenerate and allows it to maintain the levels of oestradiol and progesterone needed to maintain the pregnancy. The hCG is the hormone that allows for a pregnancy diagnosis by means of blood or urine test.
On the ninth day after ovulation the embryo is firmly implanted in the endometrial epithelium and will then develop progressively until birth. If fertilisation does not take place the Estradiol and the progesterone level in the blood will decrease quickly. In this case the hypophysis reacts by secreting FSH and a new cycle begins.
