Fertilization (Meaning and Explanation)

We explain what fertilization is, what its phases are like and when it is internal or external. Also, human fertilization and conception.

What is fertilization?

In biology, fertilization is known as fusion process of reproductive cells or gametes, one from each of the parents to create a zygote from which a totally new individual will be formed. To do this, both cells (male and female) must come together and fuse their cell nuclei, intermingling their genetic material to obtain a new and entire genome.

Fertilization is a key moment of sexual reproduction, common among animals and plants, which constitutes a new individual from the contribution of its two parents. Therefore, it is the stage that follows copulation or coitus, if any, in which both individuals prepare to release their gametes. For fertilization to normally occur, it is required that both parents belong to the same species and are fertile and compatible.

The details regarding how this process occurs can vary substantially from one species to another, occurring inside or outside the female body, for example, or even there being species capable of self-fertilization without the participation of the other sex. However, most of the time fertilization involves several well-differentiated stages, which are:

Phase 1: contact between gametes occurs. The sperm are chemically attracted to the egg, towards which they move using their flagella. Once together, a chemical recognition occurs that confirms (or not) their compatibility and the minimum conditions for union. Only then does the penetration of the substances that surround the egg (the corona radiata) occur, thanks to the joint action of the sperm.
Phase 2: penetration of the egg occurs. This penetration occurs thanks to the rupture of the zona pellucida of the egg through the contents of the acrosome, the small deposit of hydrolytic proteins at the tip of each sperm, and concludes as soon as one of them penetrates into the interior of the egg (or oocyte). , leaving out its scourge. Generally, only one sperm manages to penetrate the egg.
Phase 3: fusion of cell nuclei. Once the oocyte is penetrated, the acrosome reaction takes place, by which the sperm fuses its plasma membrane with that of the egg, allowing its contents to spread within the latter, and for the two cell nuclei to meet. Then a dynamic of nuclear and genetic fusion occurs, in which each cell contributes half of the genome of each parent, to form a complete DNA: that of a new individual.
Phase 4: formation of the zygote. Once the nuclei have fused, the zygote is ready to begin its growth and the multiplication of its contents, thus beginning the creation of a new individual equipped with its own cells.

Internal and external fertilization

Two types of fertilization are usually distinguished in animals, depending on where the gametes meet: internal and external fertilization.

internal fertilization. It occurs within the body of the female parent, in the uterus or its surroundings. To do this, copulation must first occur, in which the male parent penetrates the female's body to inject his sperm inside. The zygote then attaches to the mother's uterus (in the case of viviparous animals) or is expelled in the form of an egg (in the case of oviparous animals). Human fertilization or the fertilization of birds are clear examples of these two trends.
External fertilization. It occurs in the environment, in which both parents release their gametes, in order to promote contact. The zygotes thus produced can then adhere to rocks, plants or even be carried on the body of one of the parents, depending on the species and its strategies to guarantee the survival of the offspring. This type of fertilization is typical of aquatic animal species, especially invertebrates.

Fertilization and conception

The terms “fertilization” and “conception” are often used synonymously. However, in more specialized fields, a difference between the two can be recognized, according to which it is preferable to use “fertilization” for the entire process that goes from the release of the gametes to the production of the zygote.

On the contrary, “Conception” is restricted to the moment in which a sperm penetrates the egg and triggers the set of cellular changes that lead to the production of an embryo.

human fertilization

Fertilization in the case of human beings It is sexual, internal and crossed (that is, each parent contributes a type of gametes), and it takes place in the fallopian tubes, within the female reproductive system. That means that intercourse and the release of the sperm that reach the uterus must have occurred first, that is, sexual intercourse. In this it is similar to that of all known mammals.