GERM CELLS: CHARACTERISTICS, FORMATION, TYPES, MIGRATION - BIOLOGY - 2025

The germ cells are the precursor cells of gametes in animals with sexual reproduction. They are embryonic precursors that differentiate from multiple somatic cell lineages (soma = body) very early in the development of most species.

Virtually all organisms that reproduce sexually arise from the fusion of two gametic cells. Gametes are specialized cells that carry half of the genetic information of the individual that produces them, the father and the mother (they are haploid cells).

Human embryonic stem cells in cell culture. Ryddragyn at English Wikipedia

All gametes produced by an animal are formed from a special line of cells known as the germ line, which develop according to a complex set of specific signals. These cells represent the main "transfer" pathway of the genome and cytosolic components from one generation to another.

Germ cells are responsible for the processes of speciation and evolution, since it is the changes that occur in these that are transmitted from one generation to the next. Additionally, these cells are the ones that mediate the transmission of hereditary diseases from parents to their children, especially in humans.

Germ cell characteristics

Germ cells are "pluripotent" or "totipotent" embryonic cells, that is, they can differentiate into almost any cell type under the right conditions and signals. In addition, they are competent cells for their "self-renewal", as they are responsible for their own regeneration.

These cells are the only ones capable of producing gametes, which are the cells that can form a new organism, a property that the other cells of an embryo lose as they differentiate.

Some authors consider them, then, as the "stem cells" of a species, since they do not form organs but rather new individuals. Likewise, these cells are the main means by which species evolve and are the vehicles for the transmission of hereditary diseases, especially in humans.

Germ cells produce gametes through processes known as meiosis and gametogenesis (oogenesis and spermatogenesis in many animals), which are characteristic and unique to this group of cells.

Seminiferous tubules with mature sperm. Nephron

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Germ cells are distinguished early from other somatic cell lines during embryonic development.

How does it happen in some invertebrates?

Fruit fly Drosophila melanogaster. Taken and edited from: Sanjay Acharya

In many species, such as the fruit fly D. melanogaster, these cells are formed from primordial cells of the blastula that have “inherited” a cytosolic determinant known as “germplasm” or “germ plasm”, that is, very particular blastomeres.

Said germplasm contains structural elements and some messenger RNA and during oogenesis and fertilization, it undergoes different cytosolic movements, to later form primordial cell clusters in the blastula stage, which will give rise to primordial germ cells.

The blastomeres that have the "germ plasm" divide asymmetrically, inheriting the germplasm to a single daughter cell. When the embryo reaches the gastrula stage, then the equivalent division of these cells begins and the population of primordial cells in the germ line expands.

How does it happen in humans?

In mammals like man, on the other hand, the involvement of a “germ plasm” has not been reported during the process of primordial germ cell formation, but rather the specification of this line is determined by cell-cell interactions.

Primordial germ cells, during the early stages of embryogenesis, are located in a kind of extra-embryonic compartment and, in humans, this occurs around the third week of development.

Once the line of primordial cells is defined, they migrate towards the female or male gonads, where the oogenesis or spermatogenesis processes are activated, respectively.

The interaction of the primordial cells with the somatic cells of the gonads, in addition to the presence of sex chromosomes and other maternal factors, is what defines the determination of sex in the germ line, although this process can vary considerably between a species and another.

Differential gene expression

For the initial "separation" of somatic cells and germ cells to take place, the first thing that occurs is a differential expression of genes, since in the germ line the characteristic genes of somatic lines are repressed in order to start the "program Germ cell genetics.

During their formation process, these cells also migrate from where they originate to the specific place where the gonads will form, which are the gamete-producing tissues in the adult.

Cell migration is also achieved through the activation of a whole migratory "machinery" and different "guiding" mechanisms, which have to do with multiple genetic and epigenetic factors (which do not have to do with the modification of the nucleotide sequence).

Migration

The primordial germ cells, those that give rise to the “true” germ cells, form far from where they develop and, to establish themselves, must migrate to the sites where the ovaries and testes are located, which are the female and male gonads..

Primordial germ cells are initially seen during gastrulation as a cluster of cells at the base of the allantois, which is an extra-embryonic membrane that forms as an outflow from the primary digestive tract of the embryo.

Gastrulation of a diblastic animal. 1-Blastula. 2-Gastrula. Pidalka44

In this stage, primordial cells acquire a polarized morphology and some experiments have shown that they extend long processes as they are mobilized.

Later, these become evident in the hindgut and then emerge from the intestinal dorsum and migrate laterally, colonizing the genital ridges.

As the primordial cells move from the hindgut to the surrounding connective tissue, the latter lengthens, forming the intestinal mesentery (the tissue that lines the small intestine and connects it to the abdominal wall), a process that occurs while cells emerge through the wall of the intestine.

Control mechanisms

The arrival of the precursor cells to the gonadal tissue is controlled by the somatic cells of these structures, which apparently exert a "chemoattractant" effect on the former.

The expression of a gene known as fragilis has been shown experimentally to have much to do with the development of motility in primordial germ cells.

This gene is involved in the cell-cell adhesion processes and in the control of the cell cycle, so it is suspected that the regulation of the adhesion processes could be crucial for the initiation of the migratory process.

However, some authors consider that the migration route of these cells is controlled at the environmental level, instead of being an autonomous process.

On their way to the gonads and once in them, these cells multiply by mitosis, generating multiple clones that manage to increase the number of individuals in the cell population.

Germ cell types

Once the primordial germ cells reach their definitive tissues, they develop into female germ cells or male germ cells, as the case may be and the endogenous and exogenous signals they receive.

The female gonads are the ovaries and the male gonads are the testes. Once in these tissues, the primordial cells multiply rapidly, but the patterns of this mitotic proliferation differ between the two.

There are, then, two types of germ cells known as oogonia and spermatogonia.

Female germ cells: oogonia

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

Oogonia are mitotically active cells. They divide intensely during embryonic development, specifically from the second to the fifth month of pregnancy in humans, thus forming up to more than 7 million of these cells, although some naturally degenerate.

These cells do not divide by mitosis again in the postnatal stages, but instead differentiate periodically. During the last stages of fetal development, however, they begin to divide by meiosis, a process that remains in "arrest" until the beginning of puberty.

Male germ cells: spermatogonia

The proliferation of spermatogonia is somewhat different from that of oogonia, because despite the fact that they begin to form and multiply in the embryonic testes, they maintain their ability to divide throughout practically the entire postnatal life.

The seminiferous ducts of the testes are internally bounded with germinative spermatogonia and some of the populations composed of these cells divide by mitosis. At the beginning of puberty, groups of spermatogonia (primary spermatocytes) begin to divide by meiosis to form the secondary spermatocytes that will give rise to haploid spermatids.

Mutations

Germ cells are the "factories" where the "vehicles" for transmitting information from one generation to the next are produced. Likewise, these cells are of great importance for evolutionary processes, since almost any modification they undergo will be imprinted on the offspring.

We can say that the DNA of all cells in an organism is prone to mutations and although mutations in somatic cells are important in the context of many diseases and other conditions, they do not always extend beyond the life span of the organism. individual who carries them.

Germline mutations, on the other hand, contribute directly to evolutionary genetic processes, since these changes can be passed from one generation to the next through gametes and zygotes.

For these reasons, germ cell mutations can only be visible in the progeny and this depends, very often, on the homozygosity or heterozygosity of the affected genes in each parent.

The causes of germline mutations are many, as they can occur in response to endogenous or exogenous signals. Some of these mutations produce diseases that can be inherited through the maternal line or through the paternal line, depending on the case.

Germ cell tumors

The uncontrolled division of cells in almost any tissue in the human body, as well as in other animals, can result in the formation of tumors, which can be benign or malignant.

Those that arise from germ cells are usually called neoplasms and can be:

- Germinomas

- Teratomas

- Embryonal carcinomas

- Endodermal sinus tumors

- Choriocarcinomas

These tumors can occur regularly in the internal regions of the gonads, although they can also be related to the proliferation or aberrant migration of the primordial germ cells, which implies that they can appear in different areas of the body.

Electron micrograph of an intratubular germ cell neoplasm in the testes or 'seminoma' (Source: Nephron via Wikimedia Commons)

Tumors associated with the primordial germ cell line are known as germinomas, while embryonal carcinomas are those that are derived from embryonic "stem" cells or derived cells.

Usually, the primordial germ cells that form in extragonadal sites are shed, but teratomas are aberrant growths of extragonadal germ cells that have managed to survive and are made up of random mixtures of differentiated tissues such as cartilage, skin, hair, or teeth.

Endodermal sinus tumors are those that are formed from cells derived from extraembryonic tissues and that are differentiated, which form the endodermal yolk sac. If, on the contrary, the tumor forms in the trophoblastic layer, it is called choriocarcinoma.

Germline tumors of the ovaries account for about 20% of all ovarian tumors, are common in girls and young adults up to about 20 years of age, and are almost always teratomas of a malignant nature.

Among these, dysgerminomas have been distinguished, which are solid and fleshy tumors that have a soft cover, composed of aggregates of cells with a polygonal appearance, with prominent plasma membranes and a large number of cytosolic granules.

References

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