What is syngamy or fertilization?

Syngamy or fertilization is called fusion of two haploid gametesone male (sperm) and one female (egg or ovum), which takes place in the sexual reproduction and that marks the beginning of the development of a new individual. The term fertilization is also frequently used as a synonym.

Syngamy can occur very differently from one species to another, but it is always characterized by recognition between gametes, interaction between them to fuse their genetic material, and formation of the diploid zygote that will give rise to the embryo.

The formation of gametes occurs by meiosis, a type of cell division that produces random segregation of genes from each parent, resulting in each gamete being unique in terms of its genome. By uniting these gametes, the resulting zygote will be equally unique, greatly increasing genetic variability regarding asexual reproduction.

In the human being, for example, each individual has 23 pairs of chromosomes, 22 without counting the sex chromosome, so that 17.6 × 10 can be formed¹² different zygotes without taking into account the possible chromosomal crossing over. If chromosome crossover is taken into account, the possible different zygotes increase to 309×10²⁴.

types of syngamy

Syngamy can be classified according to multiple criteria, the three most used being the morphological similarity between the participating gametes, the parents involved and the environment in which fertilization takes place.

Depending on the morphology of the gametes, syngamy can be:

• isogamy: the two gametes are structurally and morphologically the same. It is not a very common type of syngamy, it only occurs in some protozoa.
• anisogamy or heterogamy: each of the two gametes that participate in fertilization is different, one male and one female, both in structure and in size and morphology. It is the most common type of syngamy.
• oogamy: the male and female gamete are very different, the male being generally small and mobile and the female large and immobile. In addition, the female gamete provides the nutrients and reserves that the formation of the zygote will need.
• hologamy: when two organisms act at the same time as gametes. For example, some yeasts.

According to the parent individuals involved:

• Allogamy or cross fertilization: each gamete comes from different individuals.
• Autogamy or self-fertilizationBoth gametes come from the same individual. For example, in hermaphroditic flowers of angiosperms; It can happen, as in this case of hermaphroditic flowers, that autogamy and allogamy occur at the same time, that is, that an individual has flowers fertilized with pollen from the same plant and flowers fertilized with pollen from other plants of the same species.

According to the medium in which it takes place:

• external: fertilization or external syngamy is characteristic in animals and aquatic organisms. It can be of two types, with the release of free sperm and eggs into the aquatic environment, or with eggs that remain attached to the outside of the female.
• internalIn internal syngamy, fertilization occurs inside the female's body. The spermatozoa that participate in this type of syngamy, known as intraspermatozoa, are introduced into the female's body through the male's copulatory organs. Internal fertilization can occur but external embryonic development; For example, in many oviparous animals, fertilization occurs inside the female, which later deposits the fertilized egg in the external environment.

Syngamy in mammals and the human species

Fertilization in all mammals, including the human species, is a internal syngamy which occurs after copulation. During copulation, the male introduces a large number of motile sperm into the vagina and they move inside to meet the eggs.

Fertilization itself includes a phase of recognition and interaction between sperm and eggs. Both gametes must be recognized as belonging to the same species and interact with each other to fuse so that a single sperm fertilizes the same egg. The process can be summarized in these steps:

1. Attraction: It is produced by several mechanisms of chemotaxis, rheotaxis, and thermotaxis. Sperm move in response to factors such as temperature gradients between the oviduct and the ampullary portion of the uterine tube (~2°C) and concentration gradients of progesterone and other chemotactic substances such as N-Formylmethionyl-leucyl-phenylalanine (fMLF) .
2. Accession: the sperm attaches to the egg using the zona pellucida, a protein coat that surrounds the egg. Zona pellucida proteins intervene in recognition and adhesion, acting as receptors for spermatozoa and activating the acrosomal reaction; In mice, interactions between zona pellucida glycoproteins, such as ZP3, and sperm membrane proteins, such as galactosyltransferase-1 (GalT-1) and SED1, have been observed.
3. acrosome reaction: interaction between sperm membrane proteins and zona pellucida proteins activate sperm transmembrane G proteins that open calcium channels and cause exocytosis of the sperm acrosome, a vesicle with various enzymes that break down zona pellucida proteins and allow the sperm to cross it until it comes into contact with the cytoplasmic membrane of the egg. Achromose mainly contains hyaluronidase, β-N-acetylglucosaminidase, and other proteases.
4. fusion of cytoplasmic membranes: The acrosome reaction also exposes other membrane proteins of the sperm that allow the attachment of its membrane to the membrane of the ovum. In this interaction, among other possible molecules, receptors of the ovule's CD9 type intervene. As soon as the fusion of both membranes begins, the egg releases granules that contain enzymes that prevent the union of other sperm.

After fertilization, which culminates in the fusion of the cytoplasmic membranes of the two gametes, the phase of egg activation and the fusion of the haploid nuclei of each gamete to form the diploid nucleus of the zygote. Once the zygote is formed, it enters mitosis to form the morula and start embryonic development.