Germ line Embriology, Genetics and Physiology

Gametogenesis is the process of developing the two mature sex cells, spermatozoon (sperm) and ovum (oocyte), which are necessary for the formation of a human being through fertilization. Spermatozoa mature in the testicles, while ova mature in the ovaries. Both sex cells undergo the same stages of multiplication, growth, and maturation.

These gametes contain encoded information specific to the individual, which is transmitted through biochemical means. This information is stored in the membrane and cytoplasm of the gametes and influences the subsequent development of new human functions.

During fertilization, the germ cells involved go through cellular transformations, following certain rules. Firstly, all human somatic cells have a diploid number of chromosomes, which means they have 46 chromosomes grouped in pairs of two. However, in order to prevent offspring from having double the number of chromosomes compared to their parents, male and female germ cells must reduce their chromosome count to a haploid set of 23 chromosomes through meiotic division.

In the process of gametogenesis, male germ cells transform from voluminous and round cells into cells with a head, neck, and tail, gaining mobility. On the other hand, female germ cells start small and increase in size due to an increase in cytoplasm volume, reaching a diameter of approximately 150 micrometers. Female germ cells remain immobile.

During the first meiotic division, homologous chromosomes pair up and exchange genetic material. In the second meiotic division, the divided cells do not replicate their DNA, resulting in each cell having a haploid number of chromosomes. This leads to the formation of gametes with 22+X or 22+Y chromosomes.

The germ line is a cellular line that originates from the endoderm of the yolk sac. These cells, known as primordial germ cells, migrate through the mesoderm of the digestive tube to the posterior wall of the embryo. During migration, germ cells multiply through mitosis, and some may remain along the migration route, potentially forming sites for tumor formation.

Migration of the germ cells © Wikimedia Commons - Ioannaclark
*PGCs - Primordial Germ Cells

Once in the presumptive region, germ cells stimulate the adjacent celomic epithelium of the mesonephros to proliferate and form primitive sex cords. These sex cords, located on the antero-internal side of each mesonephros, give rise to the genital ridges, which are the primitive gonads. The sex cords cover the primordial germ cells and contribute to the development of ovarian follicles in females and Sertoli cells in the seminiferous tubules in males.

The sex cords are crucial for the development of germ cells. Germ cells that are not covered by sex cord cells may undergo premature meiosis and degenerate. Similarly, if germ cells do not reach the presumptive gonadal region, neither sex cords nor gonads will develop, as they play an essential role in the appearance of the genital ridges.

Gametogenesis is a process that involves meiosis and cytodifferentiation, which transforms germ cells into male and female gametes. In both males and females, primordial germ cells within the gonads undergo mitotic divisions and then proceed to gametogenesis, where they develop into mature gametes (spermatozoa and oocytes). However, the process of gametogenesis differs between males and females.

In males, primordial germ cells remain dormant from the 6th week of development until puberty. During puberty, the seminiferous tubules mature, and the germ cells differentiate into spermatogonia. Through meiotic division, the sex cells mature, and spermatozoa are continuously produced from puberty until death.

In females, primordial germ cells undergo multiple mitotic divisions after being surrounded by sex cord cells, resulting in the formation of oogonia. The entire process of meiotic division begins during the 5th month of intrauterine development. All sex cells enter a dormant stage during the initial phase of meiosis and remain in this stage until sexual maturity.

Once puberty is reached, a few cells initiate gametogenesis each month in response to the monthly secretion of gonadotropic hormones from the pituitary gland. Typically, one primary oocyte matures into a secondary oocyte and is eliminated monthly. This oocyte enters the second phase of meiotic division and completes meiosis upon fertilization. These monthly cycles continue from puberty until menopause, which occurs around the age of 50.

Meiosis is a cell division process that occurs exclusively in the germ line. In mitosis, a diploid cell divides once to produce two diploid daughter cells with the same genetic material. However, in meiosis, a germ cell undergoes two successive and qualitatively different nuclear and cellular divisions to generate four haploid cells. In males, meiotic cell divisions are equal and result in the formation of four identical spermatozoa. On the other hand, in females, meiotic cell divisions are unequal and lead to the production of a single mature oocyte and three non-functional haploid polar bodies.

Although the timing of meiosis differs between males and females, the chromosomal events of the process are the same in both sexes. Primordial germ cells, like all normal somatic cells, contain 23 pairs of chromosomes, with 22 pairs being autosomes and the remaining two determining the individual's sex. The sex chromosomes are of two types: X and Y. Individuals with two X chromosomes are genetically female, while those with one X and one Y chromosome are genetically male.

Differences between Mitosis & Meiosis (Illustration of sex Chromosome Disjunction)
© Wikimedia Commons - Community College Consortium for Bioscience Credentials

The first meiotic division involves DNA replication and recombination, resulting in two haploid daughter cells. During the initial phase of meiosis, similar to mitosis, the replication of each chromosome spiral of DNA occurs, transforming the diploid cells into tetraploid cells. This marks the beginning of gametogenesis. In the next stage, called prophase, the chromosomes condense into compact, double-stranded structures. During the final stage of prophase, the double-stranded chromosomes of each homologous pair form chiasmata, allowing for genetic recombination through crossing-over. Metaphase and anaphase follow, resulting in the distribution of double-stranded chromosomes to each nucleus of the daughter cells.

In the first meiotic division, the centromeres of the chromosomes do not replicate, keeping the two chromatids of each chromosome together. The resulting daughter cell nuclei are haploid but contain the same amount of DNA as the mother germ cells. After the formation of the daughter nuclei, the cell divides, producing two secondary spermatocytes in males and one secondary oocyte and the first polar body in females.

In the second meiotic division, the double-stranded chromosomes divide, resulting in the formation of four haploid daughter cells. This division does not involve DNA replication. The chromosomes condense and align along the equator during the prophase and metaphase of the second meiotic division. The centromeres replicate, and in anaphase, the double-stranded chromosomes divide into two single-stranded chromosomes, each distributed to each daughter nucleus. In males, this division produces two definitive spermatocytes or spermatids. In females, the second meiotic division is highly unequal, generating one large definitive oocyte and one small polar body. The first polar body undergoes a second meiotic division to produce a second polar body. The oocyte enters a second phase of meiotic arrest and does not complete meiosis until fertilization occurs.

Abnormal meiotic divisions can result in gametes with an abnormal number of chromosomes, leading to chromosomal disorders such as trisomy or monosomy. Normally, the two members of a chromosome pair separate during the first meiotic division, so that each daughter cell receives one component of each pair. Nondisjunction, where the separation of chromosome pairs does not occur correctly, can cause this abnormality during meiosis. The result of chromosomal nondisjunction causes one daughter cell to have 24 chromosomes and the other to have 22 During fertilization, a gamete with 23 chromosomes fuses with a gamete with 24 or 22 chromosomes, resulting in an individual with 47 chromosomes (trisomic) or 45 chromosomes (monosomic). Nondisjunction can occur during the first or second meiotic divisions and can involve any of the chromosomes.

Ploidy refers to the number of copies of each chromosome in a cell nucleus, while the number "n" indicates the number of copies of each unique DNA strand in the nucleus. The relationship between ploidy and the number "n" is not always the same due to the replication of centromeres during meiosis. Primordial germ cells and somatic cells are diploid, containing two copies of each chromosome. Mature gametes, which have undergone meiosis, are haploid with a single copy of each chromosome. In certain stages of the cell cycle, diploid cells can have a single DNA strand on the chromosome and are called 2n. During the initial phases of meiosis or mitosis, each chromosome of a diploid cell has two DNA strands, making the cell 4n.

References

1. Anatomia Omului, Volumul I: Embriologie; Semestrul I; Prof. univ. dr. med. Andrei Motoc; Editura „Victor Babeș”, Timișoara, 2018

2. Embriologie - Sinteze didactice; Romeo Chiriac, Anca Indrei ;Editura Fundatiei Chemarea, Iasi, 1997

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