Fertilization is the term used to describe the process in which the male gamete unites with the female, which initiates the embryo development. Because it involves gametes, fertilization is a process observed only in the so-called sexual reproduction.
→ Types of fertilization
Internal fertilization: In this type, there is the junction of the male and female gamete inside the body of the one that produces the female gamete. This is the case of human beings, a species in which fertilization occurs within the female reproductive tract.
External fertilization: External fertilization takes place outside the body, in the environment. This is the case, for example, with frogs. In these animals, the male and female unite, the male embraces the female, and both release the gametes together. The sperm then fertilize the eggs, which develop in the environment. The place of development of eggs depends on the type of species.
Cross fertilization: In cross-fertilization, the fertilized gametes originate from different individuals. As an example, we can mention the chordates.
Self-fertilization: In self-fertilization, the merging gametes are produced by the same individual. An example of self-fertilization is that which occurs with tapeworms.
→ Human fertilization
In human beings, fertilization comprises the period from the meeting of the sperm with the oocyte to the fusion of the nuclei. Normally, the process takes place in the most dilated part of the fallopian tube, in a region called the ampoule.
The process of human fertilization can be divided into a few steps:
1. Sperm penetration by radiated corona, a group of cells in the ovarian follicle that surrounds the oocyte;
2. Penetration of sperm in the zona pellucida, that is, in the layer of glycoprotein that surrounds the oocyte;
3. Fusion of the sperm membrane with the egg membrane.
After sperm penetration, the egg membrane becomes impenetrable to other sperm. The oocyte, then, resumes the second meiotic division, and the female and male pro-nucleus are formed, which merge. Fertilization ensures that the diploid chromosome number of our species is restored, the sex of the individual is determined, and embryonic development begins.
By Ma. Vanessa Sardinha
Source: Brazil School - https://brasilescola.uol.com.br/o-que-e/biologia/o-que-e-fecundacao.htm