Fertilization – Concept, phases, internal, external and human

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

In fertilization, the genetic material of the parents is intermingled.

What is fertilization?

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

Fertilization is a key moment of sexual reproduction, common between animals and plants, which constitutes a new individual from the contribution of its two parents. Therefore, it is the stage that follows copulation or intercourse, if any, in which both individuals prepare to release their gametes. For fertilization to occur normally it is required that both parents belong to the same species and that they 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 are 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. Sperm are chemically attracted to the egg, towards which they move using their flagella. Once together, a chemical recognition takes place that confirms (or not) their compatibility and the minimum conditions for the union. Only then does the penetration of the substances that surround the ovule (the radiated crown) occur, thanks to the joint action of the sperm.
  • Phase 2: penetration of the ovum occurs. This penetration occurs thanks to the rupture of the zona pellucida of the ovule by the contents of the acrosome, the small deposit of hydrolytic proteins at the tip of each sperm, and ends as soon as one of them penetrates into the ovule (or oocyte) , leaving out its scourge. In general, only one sperm can penetrate into the egg.
  • Phase 3: fusion of cell nuclei. Once the oocyte has been penetrated, the acrosomic reaction takes place, through which the sperm fuses its plasma membrane with that of the ovum, allowing its content to spread within the latter, and the two cell nuclei to meet. Then there is a dynamic of nuclear and genetic fusion, 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 content, thus beginning the creation of a new individual endowed with its own cells.

Internal and external fertilization

external fertilization
In external fertilization the gametes are released into the environment.

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. For this, a copulation must first take place, in which the male parent enters the female’s body to inject his sperm inside. The zygote then attaches to the maternal uterus (in the case of viviparous animals) or is expelled as an egg (in the case of oviparous animals). Human fertilization or 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 such a way as 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 settings, a difference between the two can be recognized, according to which it is preferable to use “fertilization” for the entire process from gamete release to zygote production.

Conversely, “Conception” is restricted to the moment a sperm enters the egg and triggers the set of cellular changes that lead to the production of an embryo.

Human fertilization

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