Symptoms Ovulation
Eggs, also known as egg cells or oocytes, are the gametes of the female body. The word gamete comes from the Greek gamein (unite), indicating the reproductive function of these cells; and it is precisely from the union of an egg with a male gamete (spermatozoon) that every new life begins.
Until puberty the follicles remain dormant and to a large extent even degenerate (follicular atrèsia). Starting from this age, every 4 weeks a follicle is brought to complete maturation, together with the oocyte it contains. The oocyte is separated from the follicular cells by a thick pellucid membrane, glycoprotein, which mediates trophic exchanges.
To indicate the cyclical alternation of maturational and degenerative events of the ova, we speak of the ovarian cycle, chronologically correlated with the menstrual cycle (which reflects the changes in the uterine mucosa in response to ovarian hormones).
As mentioned, each cycle lasts about 28 days and includes a proliferative phase, which leads to the maturation of the oocyte and the follicle that contains it, an ovulatory phase in which the egg cell is released, and a post-ovulatory phase during what remains of the follicle after the explosion (dehiscence) is transformed into corpus luteum. This structure has the function of secreting progesterone, a hormone essential to allow nesting, that is, the complete and progressive penetration of the fertilized egg into the mucosa that lines the uterine cavity (called the endometrium).
It is important to remember that:
the oocyte has a maximum life of 12-24 hours, while the spermatozoa survive inside the tubes for 2-4 days. The rapid deterioration process of the egg cell stops only if fertilization occurs.
Ovulation generally coincides with the middle of the cycle, that is 14 days after the start of the last menstruation. However, while, the time between the start of ovulation and the start of the next menstruation is almost constant (14 days because hormonal events are strictly controlled in cascade), the time required to bring the ovum to complete development and release is very variable. Ovulation, therefore, does not always coincide with the 14th day of the cycle, but can be anticipated or especially extended even for several days.
At the beginning of the ovarian cycle, the maturation process involves several follicles, but usually only one reaches complete development and is expelled from the ovary to be eventually fertilized. The remaining follicles rapidly regress, according to a degenerative process that first affects the oocyte and then the follicular cells that surround it. The latter will be replaced by connective tissue.
During the maturation process of the oocyte also the follicle undergoes modifications, which lead it to support, from the endocrine point of view, the maturation of the ovum enclosed in it. After ovulation, the egg cell is readily captured by the fimbriae of the tube and channeled into it. At this level, the delicate liquid currents - associated with the peristaltic and cilia movements - push the egg towards the uterine cavity.
In the fertile period of life, approximately from 12 to 45 years, each woman will release approximately 400-450 mature egg cells, while all the other follicles will spontaneously atrophy until complete exhaustion and then menopause.
In principle, the eggs are produced alternately by each of the two ovaries. The simultaneous maturation of two or more oocytes is rare, but still possible. If fertilized, these eggs can give rise to two or more embryos.
If the egg cell is not fertilized, within ten days the corpus luteum stops producing endocrine and regresses, forming a very small scar on the ovarian surface (corpus albicantus). The rapid drop in progesterone levels, typical of the involutional phase, occurs around the 24th day of the cycle and precedes the menstrual phase (more information in the articles dedicated to the menstrual cycle and menstruation).
Fertilization of the egg
The egg is a large cell (100-150 microns in diameter), with the cytoplasm rich in reserve phospholipid materials (calf granules or yolk). After being released into the abdominal cavity, the "egg is promptly" sucked "from the respective fallopian tube, the seat for fertilization. This generally occurs in the third closest to the ovary, where the mature oocyte is joined by the spermatozoa.
For fertilization to take place it is necessary for a sperm to penetrate inside the egg cell. This is a delicate event, since the oocyte is protected by some cells (which constitute the so-called radiated crown) and membranes, such as the zone pellucida, who oppose their entry. For spermatozoa it is therefore a real obstacle race: only the first one who will be able to reach the egg and penetrate it - thanks to the release of special enzymes - will have the honor of fertilizing it.
After the entry of the sperm, the cell membrane of the oocyte undergoes a series of structural changes that prevent the entry of other semen.
Gametogenesis: the formation of egg cells
The process of formation of female gametes takes place in the embryonic ovary starting from immature cells, called oogones. These cells have a diploid chromosomal set, like all the somatic cells that make up the adult organism. After a number of mitotic divisions, oogones complete the first stage of meiosis (prophase I) before the fifth month of embryonic development.
