See also: in vitro fertilization - sterility problem - ovulation symptoms
Fertilization consists in the union of the female egg with one of the many male spermatozoa, to form a cell - the zygote - from which a new life will develop.
Role of Spermatozoa
At the height of the sexual act, the male ejaculate comes out of the penis and spills over into the upper part of the vagina, along with its sperm load. Overall, this milky liquid called sperm contains approximately 300 million spermatozoa. However, 99% of its volume (1-5 ml) is made up of liquids produced by accessory glands, such as the prostate and seminal glands. These secretions have the main function of promoting sperm motility, ensuring their nourishment and survival within the acidic environment of the vagina.
Among the first obstacles present along this path, we recall the cervical mucus, a uterine secretion, which traps the less vital spermatozoa between its meshes, not completely mature or with unfavorable morphological characteristics. This mucus becomes less hostile in the days of ovulation, that is, in the most favorable period for fertilization.
The spermatozoa that manage to escape from the mucus continue their ascent towards the upper part of the uterus, which will be followed by entry into the tuba. It has been calculated that, with the due proportions, the speed of a spermatozoon would be equal to that of a person running at 55 km / h. In reality, given the extremely small size, their advancement speed is rather low, equal to about 15 centimeters per hour (the uterus is about 6-9 cm long and the uterine tubes are about ten cm).
Fertile Period
As a rule, fertilization occurs on the same day that the egg cell is released by the ovarian follicle (around the 14th day of the canonical 4-week ovarian cycle). In fact, the mature oocyte survives for up to 24 hours after liberation. On the other hand, the spermatozoa deposited by the male can withstand up to 4 days in the crypts of the cervical mucosa and from here gradually rise towards the tubes. Normally fertilization occurs in their distal part, ie in the third closest to the ovary.
Sperm encounter - egg cell
Although fertilization puts millions of spermatozoa into play, only one of them can fertilize the egg. The latter, in fact, is protected - albeit labile - by a layer of cells called corona radiata .
After having overcome this first obstacle, the spermatozoon is in front of another, much more difficult, glycoprotein barrier, represented by the zona pellucida . In order to cross it, spermatozoa release powerful enzymes contained in the acrosome, a vesicle confined to the upper part of their head.
The process, called the acrosomal reaction, allows the sperm to dig themselves into a small canal for the fertilization of the egg. As mentioned several times, this privilege will be given only to the first spermatozoon that completes the assault on the oocyte. The fusion of the two cell membranes is very important because:
- stimulates the egg to complete its second meiotic division
- opens a way that allows the sperm nucleus to reach the oocyte nucleus and merge with it;
- triggers a chemical reaction, called cortical reaction, which prevents the fertilization of the egg by other spermatozoa (prevents polyspermy).
Zygote and Embryo
The union of the nuclei originates a new cell, called zygote, of 46 chromosomes, 23 of which are inherited from the paternal sperm and 23 of which from the maternal egg cell. The zygote will therefore encounter a long series of mitotic division, already begun in its approach towards the uterus, in which it nestles after about a week. the continuation of embryonic development is illustrated in this article: development of the fetus embryo after fertilization.
What we want to emphasize here is that sexual fertilization, as it happens in man and in many other higher organisms, allows the new individual to inherit a combination of paternal and maternal chromosomes among the possible millions.
All this, besides explaining why each of us is unique, allows the strengthening of the species, since it is at the base of natural selection, that is to say of that process that favors organisms endowed with the most suitable characters in a given environment. These characters, innate following small random mutations, are inherited, that is, transmitted to descendants through, precisely, sexual fertilization.
