WHAT ARE OOGONIA? - BIOLOGY - 2025

The oogonia are diploid germ cells are female. They are found in the ovary, grow, and are morphologically modified. In oogonia the first meiotic division occurs and through changes the female gametes or ovules originate. They are cells with the shape of spheres and the genetic material of the nucleus is particularly loose.

In us humans, the female fetus begins to form oogonia. That is, the oocytes formed at this stage represent the entire amount that will be available throughout the reproductive life of said individual.

Oogonia. Source: Chassot AA, Gregoire EP, Lavery R, ​​Taketo MM, de Rooij DG, et al.

The meiosis process stops at the secondary oocyte stage until the hormonal stimuli of puberty cause the oocyte to shed during each of the menstrual cycles.

The analogous cell in the male counterpart is spermatogonia, cells that colonize the testes. Both germ lines seek to generate haploid sexual gametes that will combine in the event of fertilization, to give rise to a diploid zygote.

Morphology of oogonia

Oogonia are precursor or germ cells that are responsible for producing oocytes: the female gametes.

These cells are found in the ovaries of human females and are spherical in shape. The nucleus of the oogonia allows them to differentiate them from somatic cells, which generally accompany them in the ovaries. These cells are called follicular and form the primary follicle.

The genetic material within the oocytes is dispersed and the nucleoli are prominent and easily distinguishable, while in somatic cells it is much more condensed.

The cytoplasm is similar to follicular cells. Some organelles, such as the endoplasmic reticulum, are poorly developed. In contrast, the mitochondria are large and prominent.

Oogenesis

Oogenesis is the process of gamete formation in female individuals. This process starts from the female germ cells, the oogonia.

The end result is four haploid daughter cells, of which only one will develop into a mature ovum and the remaining three degenerate into structures called polar bodies. We will now describe the oogenesis process in detail:

Mitotic divisions in the uterus: multiplication phase

The ovaries are the structures that make up the female reproductive system. In humans they are found as paired organs. However, they are quite variable in the animal kingdom. For example, in some viviparous fish the ovaries fuse and in birds only the left ovary is formed.

Structurally, the ovary offers a peripheral mesothelial layer called the germ layer, and inside it has a reduced fibrous layer called the albuginea.

The oogonia are lodged in the ovary. During the early stages of oogenesis, the oogonia surrounds itself with somatic cells and initiates the process of division through mitosis. Let us remember that in this type of cell division, the result is identical daughter cells with the same chromosomal load, in this case diploid.

Different oogonia pursue different destinations. Many of them are divided by successive mitosis events, while others continue to increase in size and are called first-order oocytes (see growth phase). Those that only divide by mitosis are still oogonia.

The numerous mitotic divisions that oogonia undergo in this phase seek to ensure the success of reproduction (more gametes, more possibility of fertilization).

Growth phase

In the second phase of the process, each oogonia begins to develop independently, increasing its amount of nutritive material. In this step the cell acquires a much larger size, generating the first order oocytes. The main objective of the growth phase is the accumulation of nutrients.

In the event that fertilization occurs, the cell must be prepared to meet the typical protein needs of the process; during the first divisions that follow fertilization there is no possibility of synthesizing proteins, so they must be accumulated.

Ripening phase

This phase aims to reduce the genetic load of the cell in order to generate a diploid gamete. If the gametes did not reduce their genetic load at the time of fertilization, the zygote would be tetraploid (with two sets of chromosomes from the father and two from the mother).

In the fetus, germ cells can reach a maximum of 6 to 7 million in the fifth month of life. Later, when the individual is born, many cells have degenerated and these oocytes persist. In this phase, the oocytes have already completed their first meiotic division.

Unlike mitosis, meiosis is a reductive division and daughter cells possess half the chromosome load of the stem cell. In this case, the oogonia is diploid (with 46 chromosomes) and the daughter cells will be haploid (only 23 chromosomes, in the case of humans).

The structures mentioned above are in a kind of dormancy. When the time of puberty comes, the changes start again.

Second order oocytes and polar corpuscle

In each ovarian cycle, the oocytes mature. Specifically, the oocyte present in the mature follicle (at this point the genetic load is still diploid) resumes the processes of cell division and culminates in the formation of two structures called oocyte II, with haploid genetic load and a polar corpuscle.

The fate of the second-order corpuscle is to degenerate, and take the haploid charge with it.

Later, a second meiotic division begins that coincides with the event of ovulation or expulsion of the ovum from the ovary. At this point the ovary is taken up by the uterine tubes.

This second division results in two haploid cells. The ovum takes away all the cytoplasmic material, while the other cell or second polar corpuscle, degenerates. All this described process takes place in the ovary and occurs parallel to the differentiation of the follicular formations.

Fertilization

Only in the event of fertilization (union of an egg and a sperm) does the egg undergo a second meiotic division. In case the fertilization event does not occur, the ovum degenerates in appropriately 24 hours.

The second division results in a structure that allows the union of the nuclei in the male and female gametes.

References

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