Germ Cells: Definition, Mechanisms, Gametogenesis, Spermatogenesis and Induced Differentiation

It is any biological cell that gives rise to the gametes of an organism that reproduces sexually.

In many animals, germ cells originate in the primitive streak and migrate through the intestine of an embryo to the developing gonads .

There, they undergo meiosis, followed by cell differentiation into mature gametes, either eggs or sperm. Unlike animals, plants do not have designated germ cells in early development.

Instead, germ cells can arise from somatic cells in the adult (such as the floral meristem of flowering plants).

Introduction

Multicellular eukaryotes are made of two fundamental cell types. Germ cells produce gametes and are the only cells that can undergo meiosis and mitosis. These cells are sometimes said to be immortal because they are the link between generations.

Somatic cells are all the other cells that make up the building blocks of the body and they only divide by mitosis. The germ cell lineage is called the germ line.

Germ cell specification begins during cleavage in many animals or at the epiblast during gastrulation in birds and mammals. After transport, which involves passive movements and active migration, germ cells reach the developing gonads.

In humans, sexual differentiation begins approximately 6 weeks after conception. The end products of the germ cell cycle are the egg or sperm.

Under special conditions, germ cells in vitro can acquire properties similar to those of embryonic stem cells (ES). The underlying mechanism for that change is still unknown.

These modified cells are called embryonic germ cells (EGs). Both embryonic germ cells and embryonic stem cells are pluripotent in vitro, but only embryonic stem cells have demonstrated pluripotency in vivo.

Recent studies have shown that it is possible to generate primordial germ cells from embryonic stem cells.

Specification

There are two mechanisms for establishing the germ cell lineage in the embryo.

The first form is called preformist and implies that cells destined to become germ cells inherit specific germ cell determinants present in the germ plasma (specific area of ​​cytoplasm) of the ovum.

The unfertilized egg of most animals is asymmetric: different regions of the cytoplasm contain different amounts of messenger RNA and proteins.

The second form is found in birds and mammals, where germ cells are not specified by these determinants but by signals controlled by zygotic genes.

In mammals, a few cells of the early embryo are induced by signals from neighboring cells to become primordial germ cells.

Mammalian eggs are somewhat symmetrical and after the first divisions of the fertilized ovum, the cells produced are all totipotent. This means that they can differentiate into any type of cell in the body, and therefore germ cells.

The specification of primordial germ cells in the laboratory mouse begins with high levels of bone morphogenetic protein (BMP) signaling, which activates the expression of the transcription factors Blimp-1 / Prdm1 and Prdm14. .

Migration

Primordial germ cells, germ cells that have yet to reach the gonads, also known as (PGC), precursor germ cells, or gonocytes, divide repeatedly on their migration path through the intestine and into the gonads. Developing.

Invertebrates

In the Drosophila model organism, the polar cells passively move from the posterior end of the embryo to the posterior midgut due to inflation of the blastoderm. They then actively move through the intestine into the mesoderm.

Endodermal cells differentiate and together with Wunen proteins induce migration through the intestine. Wunen proteins are chemo-films that move germ cells away from the endoderm and into the mesoderm.

After dividing into two populations, the germ cells continue to migrate laterally and in parallel until they reach the gonads.

Vertebrates

In the Xenopus egg, germ cell determinants are found in the more plant blastomeres. These presumed primordial germ cells are delivered to the endoderm of the blastocele by gastrulation.

They are determined as germ cells when gastrulation is complete. Then there is migration from the hindgut along the gut and through the dorsal mesentery.

Germ cells divide into two populations and move to paired gonadal ridges. Migration begins with 3-4 cells undergoing three rounds of cell division, so that around 30 primordial germ cells reach the gonads.

In the migratory path of primordial germ cells, the orientation of the underlying cells and their secreted molecules, such as fibronectin, play an important role.

Mammals have a migration path comparable to that of Xenopus. Migration begins with 50 gonocytes and approximately 5,000 primordial germ cells reach the gonads. Proliferation also occurs during migration and lasts 3 to 4 weeks in humans.

The primordial germ cells come from the epiblast and subsequently migrate to the mesoderm, endoderm, and the posterior part of the yolk sac.

Migration takes place from the hindgut along the intestine and through the dorsal mesentery to reach the gonads (4.5 weeks in humans).

Somatic cells in the path of germ cells provide them attractive, repulsive and survival signals. But germ cells also send signals to each other.

In reptiles and birds, germ cells use another path. The germ cell-like primordial cells come from the epiblast and move to the hypoblast to form the germ crescent (anterior extra-embryonic structure).

The gonocytes are then compressed into the blood vessels and use the circulatory system for transport. They come out of the vessels when they are at the level of the gonadal ridges.

Cell adhesion on the endothelium of blood vessels and molecules, such as chemoattractants, are likely involved in the migration of stem cells from germ cells.

The SRY gene on the Y chromosome

The SRY (sex-determining region of the Y chromosome) directs male development in mammals by inducing the somatic cells of the gonadal crest to become a testis, rather than an ovary.

The sex-determining region of the Y chromosome is expressed in a small group of somatic cells in the gonads and influences these cells to become Sertoli cells (support cells in the testes).

Sertoli cells are responsible for sexual development along the male pathway in many ways.

One of these ways involves the stimulation of primordial cells that arrive to differentiate into sperm. In the absence of the Sry gene, primordial germ cells differentiate into eggs.

Removal of the genital ridges before they begin to develop into testes or ovaries results in the development of a female, independent of the carried sex chromosome.

Gametogenesis

Gametogenesis, the development of diploid germ cells into haploid ovules or spermatozoa (respectively oogenesis and spermatogenesis) is different for each species, but the general stages are similar. Oogenesis and spermatogenesis have many characteristics in common, both involving:

  • Mitosis.
  • Wide morphological differentiation.
  • The inability to survive for long if fertilization does not occur.

Despite their homologies, they also have big differences:

Spermatogenesis has equivalent meiotic divisions that result in four equivalent spermatids, while oogenic meiosis is asymmetric: only one egg is formed along with three polar bodies.

Different maturation times: ogenic meiosis is interrupted in one or more stages (for a long time) while spermatogenic meiosis is rapid and uninterrupted.

Oogenesis

After migration, the primordial germ cells will develop into oogonia in the forming gonad (ovary). The oogonia proliferates extensively by mitotic divisions, up to 5-7 million cells in humans. But then many of these oogonia die and about 50,000 remain.

These cells differentiate into primary oocytes. In week 11-12 after intercourse, the first meiotic division begins (before birth for most mammals) and remains arrested in prophase I for a few days to many years, depending on the species.

It is in this period or in some cases at the beginning of sexual maturity when the primary oocytes secrete proteins to form a layer called the zona pellucida and also produce cortical granules that contain enzymes and proteins necessary for fertilization.

Meiosis is sustained by follicular granulosa cells that send inhibitory signals through gap junctions and the zona pellucida. Sexual maturation is the beginning of periodic ovulation.

Ovulation is the regular release of an oocyte from the ovary into the reproductive tract and is preceded by follicular growth. Some cells in the follicle are stimulated to grow, but only one oocyte is ovulated.

A primordial follicle consists of an epithelial layer of granulosa follicular cells that encloses an oocyte. The pituitary gland secretes follicle stimulating hormones (FSH) that stimulate follicular growth and oocyte maturation.

The thecal cells around each follicle secrete estrogen. This hormone stimulates the production of follicle-stimulating hormone receptors in follicular granulosa cells and at the same time has a negative feedback on the secretion of follicle-stimulating hormone.

This results in competition between the follicles and only the follicle with the majority of the follicle-stimulating hormone receptors survives and ovulates. Meiotic division I continues in the ovulated oocyte stimulated by luteinizing hormones (LH) produced by the pituitary gland.

Follicle-stimulating hormone and luteinizing hormones block the gap junctions between the follicle cells and the oocyte, thus inhibiting communication between them. Most of the follicular granulosa cells remain around the oocyte and thus form the cumulus layer.

Large non-mammalian oocytes accumulate egg yolk, glycogen, lipids, ribosomes, and the messenger RNA necessary for protein synthesis during early embryonic growth.

These intense RNA biosynthesis are reflected in the structure of the chromosomes, which decondense and form lateral loops that give them a lamp-brush appearance.

Oocyte maturation is the next phase of oocyte development. It occurs in sexual maturity when hormones stimulate the oocyte to complete meiotic division I.

Meiotic division I produces 2 cells of different sizes: a small polar body and a large secondary oocyte. The secondary oocyte undergoes meiotic division II resulting in the formation of a second small polar body and a large mature ovum, both of which are haploid cells. The polar bodies degenerate.

Oocyte maturation remains metaphase II in most vertebrates. During ovulation, the arrested secondary oocyte leaves the ovary and rapidly matures into an egg ready for fertilization.

Fertilization will cause the egg to complete meiosis II. In human females there is proliferation of the oogonia in the fetus, meiosis begins before birth and remains in the meiotic division I until the 50 years, ovulation begins at puberty.

Egg growth

A large somatic cell of 10-20 µm generally takes 24 hours to double in mass for mitosis. In this way, it would take a long time for the cell to reach the size of a mammalian egg with a diameter of 100 μm (some insects have eggs of about 1,000 μm or more).

Eggs therefore have special mechanisms to grow to their large size. One of these mechanisms is having extra copies of genes: meiotic division I is paused for the oocyte to grow while it contains two sets of diploid chromosomes.

Some species produce many additional copies of genes, such as amphibians, which can have up to 1 or 2 million copies. A complementary mechanism depends in part on the synthesis of other cells.

In amphibians, birds, and insects, the yolk is produced in the liver (or its equivalent) and secreted into the blood. Neighboring accessory cells in the ovary can also provide nutritional support of two kinds.

In some invertebrates, some oogonia develop into nurse cells. These cells are connected by cytoplasmic bridges with oocytes. The nurse cells of insects provide oocyte macromolecules such as proteins and messenger RNA.

Follicular granulosa cells are the second type of accessory cells in the ovary in both invertebrates and vertebrates. They form a layer around the oocyte and nourish it with small molecules, without macromolecules, but eventually with their smaller precursor molecules, through gap junctions.

DNA mutation and repair

The mutation frequency of female germ cells is approximately 5 times lower than that of somatic cells.

Mouse oocyte in the diketic meiosis stage (prolonged diplotene) actively repairs DNA damage, while DNA repair was not detected in the pre-dictyate meiosis stages (leptotene, zygotene and pachytene).

The long period of meiotic arrest in the four-chromatid dichromate stage of meiosis may facilitate recombinational repair of DNA damage.

Spermatogenesis

Mammalian spermatogenesis is representative for most animals. In human males, spermatogenesis begins at puberty in seminiferous tubules in the testes and continues continuously.

Spermatogonia are immature germ cells. They continually proliferate by mitotic divisions around the outer edge of the seminiferous tubules, near the basal lamina.

Some of these cells stop proliferation and differentiate into primary spermatocytes. After passing through the first meiotic division, two secondary spermatocytes are produced.

The two secondary spermatocytes undergo the second meiotic division to form four haploid spermatids. These spermatids differ morphologically into sperm by nuclear condensation, cytoplasmic ejection, and acrosome and flagellum formation.

Developing male germ cells do not complete cytokinesis during spermatogenesis. Consequently, cytoplasmic bridges ensure the connection between the clones of the differentiating daughter cells to form a syncytium.

In this way, haploid cells are supplied with all the products of a complete diploid genome. Sperm that carry a Y chromosome, for example, are supplied with essential molecules that are encoded by genes on the X chromosome.

DNA mutation and repair

The mutation frequency of cells in the different stages of spermatogenesis in mice is similar to that of female germ cells, it is 5 to 10 times lower than the mutation frequency in somatic cells. Low mutation frequency is a characteristic of germ cells in both sexes.

Homologous repair of double strand break recombinations occurs in mice during sequential stages of spermatogenesis, but is more prominent in spermatocytes.

The lower mutation frequencies in germ cells compared to somatic cells appear to be due to more efficient removal of DNA damage by repair processes, including homologous recombination repair during meiosis.

The frequency of the mutation during spermatogenesis increases with age. Mutations in spermatogenic cells from old mice include a higher prevalence of transversion mutations compared to young and middle-aged mice.

Diseases

Germ cell tumor is a rare cancer that can affect people of all ages. 2.4 children out of a million are affected, and it accounts for 4% of all cancers in children and adolescents under 20 years of age.

Germ cell tumors are generally located in the gonads, but they can also appear in the abdomen, pelvis, mediastinum, or brain.

Germ cells that migrate to the gonads may not reach that destination and a tumor may grow wherever they end up, but the exact cause is still unknown. These tumors can be benign or malignant.

Induced differentiation

Inducing the differentiation of certain cells into germ cells has many applications. One implication of induced differentiation is that it may allow eradication of male and female factor infertility.

Furthermore, it would allow same-sex couples to have biological children if sperm could be produced from female cells or if eggs could be produced from male cells.

Hayashi and Saitou’s research group at Kyoto University began efforts to create sperm, skin eggs, and embryonic stem cells.

These researchers produced germ cell primordial cells (CGPs) from embryonic stem cells (EMCs) and skin cells in vitro.

Hayashi and Saitou’s group were able to promote the differentiation of embryonic stem cells into germ cell-like primordial cells with the use of precise time and bone morphogenetic protein 4 (Bmp4).

After succeeding with embryonic stem cells, the group was able to successfully promote the differentiation of induced pluripotent stem cells (iPSCs) into stem cells of germ cells.

These germ cell-like primordial cells were used to create sperm and oocytes.

Efforts for human cells are less advanced due to the fact that the germ cell-like primordial cells formed by these experiments are not always viable.

In fact, the Hayashi and Saitou method is only one-third more effective than current in vitro fertilization methods, and the primordial cells produced by germ cells are not always functional.

Furthermore, induced germ cell primordial cells are not only as effective as natural germ cell primitive cells, but they are also less effective at erasing their epigenetic markers when differentiated from induced pluripotent stem cells.

There are also other applications of germ cell induced differentiation.

Another study demonstrated that culturing human embryonic stem cells in mitotically inactivated porcine ovarian fibroblasts (POFs) causes differentiation into germ cells, as demonstrated by gene expression analysis.