The eggs, also known as egg cells or oocytes, are the gametes of the female body. The word gamete derives from the Greek gamein (to unite), to indicate 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 quiescent and largely 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 remains 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 eggs, we speak of ovarian cycle, chronologically correlated with the menstrual cycle (which reflects the variations of 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 the which remains of the follicle after the explosion (dehiscence) turns into a corpus luteum. This structure has the function of secreting progesterone, an essential hormone to allow the nesting, that is the complete and progressive penetration of the fertilized egg into the mucosa that internally covers the uterine cavity (called 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 intervenes.
Ovulation generally coincides with the middle of the cycle, ie 14 days after the beginning of the last menstruation. However, while, the time between the beginning of ovulation and the beginning of the next menstruation is almost constant (14 days because the hormonal events are strictly controlled by cascade), the time necessary to bring the egg to complete development and release is very variable. The ovulation, therefore, does not always coincide with the 14th day of the cycle, but can be anticipated or above all extended even of several days.
At the beginning of the ovarian cycle, the maturation process involves several follicles, but normally only one reaches complete development and is expelled from the ovary to be possibly fertilized. The remaining follicles regress rapidly, following 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 oocyte maturation process 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 promptly captured by the fimbriae of the tube and channeled inside it. At this level, the delicate liquid currents - associated with the peristaltic and eyelash movements - push the egg towards the uterine cavity.
In the fertile period of life, approximately from 12 to 45 years, each woman will free approximately 400-450 mature egg cells, while all the other follicles will spontaneously atrophy until complete exhaustion and then to menopause.
In principle, the eggs are produced alternately from each of the two ovaries. The maturation of two or more eggs at the same time 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 ceases endocrine production and regresses, forming a very small scar on the ovarian surface (albicant body). The rapid fall in progesterone levels, typical of the involution 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 ovule is a large cell (1-2 mm in diameter), with the cytoplasm rich in reserve phospholipid materials (calf grains or yolk). After being released into the abdominal cavity, the egg is promptly "sucked" by the respective Fallopian tube, the site designated for fertilization. This generally occurs in the third nearest the ovary, where the mature oocyte is reached by the spermatozoa.
In order for fertilization to take place, a spermatozoon must penetrate into 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, like the zona pellucida, which oppose their entry. Spermatozoa are therefore a veritable obstacle race: only the first to reach the egg and penetrate it - thanks to the release of specific enzymes - will have the honor of fertilizing it.
After the entrance of the spermatozoon, the cell membrane of the oocyte undergoes a series of structural modifications that prevent the entry of another seed.
Gametogenesis: the formation of egg cells
The process of formation of female gametes takes place in the embryonic ovary starting from immature cells, called oogoni. These cells have a diploid chromosome, like all the somatic cells that make up the adult organism. After a number of mitotic divisions, oogons complete the first stage of meiosis (prophase I) before the fifth month of embryonic development.
