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What is germ cell?

Meaning of Germinal Cell

Stem cells are embryonic cells responsible for forming the lines that will give rise to male and female gametes in adult animals, sperm and ovules respectively.

Origin of germ cells in animals

These germ cells have an extragonadal and extraembryonic origin, that is, they are not formed directly in the embryonic region of the gonads, nor in the cells that give rise to the embryo itself, but rather in their embryonic junctions.

Therefore, they are present mainly in the wall of the yolk sac near the allantois, more specifically, in the epithelium of the dorsal endoderm of this structure.

During the embryonic period of organogenesis, between the fourth and fifth week of human development, germ cells migrate through amoeboid movements, obeying chemical signaling for the release of attractive chemotactic substances, along the posterior primitive intestine. , until finding mesonephros, where the future gonads will develop.

Once the germ cells have reached the forming genital ridges, they are called oogonia in females and spermatogonia in males.

How germ cells form gametes

Germ cells divide by mitosis less frequently than somatic cells in the earliest process of embryo formation, embryogenesis.

However, when they reach their destination, after migration, they will form gametes through meiosis, a process of cell division that results in daughter cells with half as many chromosomes as the original cell.

Therefore, these are the only cells that carry out meiosis in the organism.

The gender of the individual, in the mammalian group, is genetically determined by the presence of Y and X chromosomes in males (n-XY genotype) and two X chromosomes in females (n-XX genotype). For this reason, males are said to be heterogametic, due to the XY genotype, and females, homogametic, due to XX.

Despite this genetic determination, the sex-specific development of germ cells such as sperm or oocytes depends on the induction of signals released by gonadal cells, which is detailed a little later.

Then, during embryogenesis, the germ cells of an ovary enter meiosis and are already on their way to oogenesis, which is completed throughout the woman’s life after puberty.

In contrast, germ cells in a testicular environment do not enter meiosis until puberty.

Retinoic acid is a signaling molecule that controls the biochemical decision to enter meiosis in the testicular milieu that is provided by the mesonephric ducts and tubes. The presence of retinoic acid induces chromosome condensation and the entry into meiosis of undifferentiated germ cells.

How the testicular or ovarian environment is formed

In the fifth week of human development, the gonadal ridges appear, which are a thickening of cells near the mesonephros.

If in the cells of the region of the gonadal crest, and in the others, the genetic material contains the Y chromosome, then the rudiment of the gonads will become a testicle and the germinal cells in spermatogonia, organizing themselves in the medullary region. This is due to the expression of signaling substances that mark these germ cells.

The main gene present on the Y chromosome, responsible for sexual differentiation, is the SRY (for sex-determining region Y), also known as the TDF (testis-determining factor) gene.

In the genital ridges of males, the primordial germ cells are surrounded by somatic supporting cells, the Sertoli cells, and form with them solid cords called the seminiferous cords.

In this period of development, the seminiferous tubules are dense structures made up of clear spermatogonia interspersed with the basal part of Sertoli cells.

When there is no Y chromosome, the rudiment will mature in the ovary and become populated with ovules, derived from germ cells, in the cortical region. For this reason, the female sex is considered the “natural sex”, since it occurs in the absence of the SRY gene, so it does not need a specific genetic beginning for its differentiation, as in the case of the male.

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