Ответы
1. The first stage (denudation)
The egg is covered with a layer of follicular cells, forming the so-called radiant crown. To reach the outer layer of the egg, sperm must overcome this barrier.
They are actively introduced between the individual cells of the crown, secreting special enzymes (hyaluronidase). Under the influence of these enzymes, the intercellular substance connecting the cells of the crown liquefies. Moving along the oviduct, the egg rubs against the villi covering its walls, due to which scattering of follicular cells occurs. For successful fertilization, there is no need for complete denudation. It is enough that the gum to reach the outer layer of the egg.
2. The second stage
Sperm reach the membrane of the egg and penetrate through it into the zone of the pellucide (near vitelline space). At the same time, the final maturation of the egg occurs. From the nucleus, which contains the maternal genetic material, a female pronucleus is formed.
3. The third stage
One, in rare cases, several sperm pass through the protective vitelline membrane and penetrate into the cytoplasm of the egg. In this case, only the sperm head enters the cytoplasm, which undergoes certain changes and turns into a pronucleus containing the genetic material of the father.
4. The fourth stage
The female and male pronuclei that have completed their formation are attracted to each other and merge, completing the fertilization process. At this moment, a unique genetic code arises from the two sets of chromosomes, which determines the sex, color of hair, eyes and many other characteristics of the new organism. The resulting cell (zygote) gives rise to a new human life.
Unfortunately, not all couples manage to conceive a child in a natural way. The technologies of artificial insemination (IVF, ICSI, etc.) come to the rescue, which make it possible to deal with some forms of female and male infertility.