Gametogenesis. Reproductive cells

Gametogenesis is the production of specialized germ cells, or gametes. During the gametes’ maturation the number of chromosomes is reduced by half to the haploid number.

Primordial germ cells, lso known as PGCs, precursor germ cells or gonocytes or gonoblasts, in human embryos appear in the wall of the yolk sac at the end of the third week of the prenatal life. These cells are indifferent: they are not divided onto the male and the female cells. They migrate by ameboid movement from the yolk sac toward through tthe developing gonads (the paired gonadal ridges on the medial surface of the mesonephros), divide repeatedly on their migratory route, and differentiate there into spermatogonia or oogonia (in humans, sexual differentiation starts approximately 6 weeks post conception). Germ cells produce gametes and are the only cells that can undergo meiosis as well as mitosis.

Spermatogenesis

Male reproductive cells, the spermatozoa, are produced in seminiferous tubules of the testis beginning with the puberty age and continue throughout adult life. The process of differentiation of the male germ cells is called spermatogenesis and can be divided into 4 phases (Fig.2):

• proliferation phase

• growth phase

• maturation phase

• formation, or spermiogenesis

The phase of proliferation. During this phase spermatogonia divide by mitoses, producing successive generations of cells that finally give rise to primary spermatocytes. Spermatogonia consist of two types: type A spermatogonia, and type B spermatogonia (fig.1).

• Type A dark spermatogonia are true stem cells, steady to harmful factors action, rarely dividing cell. At sexual maturity, this cell undergoes a series of mitoses, and the newly formed cells can follow one of 2 paths: they can continue, as undifferentiated stem cells (type A pale spermatogonia), or they can differentiate during progressive mitotic cycles to become type B spermatogonia.

• Type B spermatogonia are committed cells, they prepare to the first meiotic division, enter the phase of growth and give rise to primary spermatocytes.

The phase of growth. During this phase primary spermatocytes grow (their size 4 times more as spermatogonia), and enter a prolonged prophase of the first meiotic division (lasts about 22 days), followed by rapid completion of meiosis I and formation of secondary spermatocytes;

The phase of maturation. Secondary spermatocytes immediately enter the second meiotic division, to form spermatids, which contain the haploid number of chromosomes;

The phase of formation, or spermiogenesis. During this phase the spermatids go through an elaborate process of cytodifferentiation, producing small, highly motile spermatozoa.

Spermiogenesis is a complex process of differentiation that includes (Fig.3):

• formation of the acrosome;

• condensation and elongation of the nucleus;

• flagellum’ development;

• the loss of much of the cytoplasm.

The acrosome is a special organelle, originated from the Golgi complex. It is a membrane-limited vesicle (the acrosomal cap), which cover the anterior half of the condensing nucleus. The acrosome contains several hydrolytic enzymes, such as hyaluronidase, penetrase, acrosin (trypsin-like e nzyme), acid phosphatase, and proteases. The acrosome thus serves as a specialized type of lysosomes. These enzymes are known to dissociate follicular cells of the corona radiata and to digest the zona pellucida, structures that surround recently ovulated eggs.

Fig.3. Top: The principal changes occurring in spermatids during spermiogenesis.

Bottom: The structure of a spermatozoon.

During spermiogenesis the centrioles of the spermatid migrate to a position near the cell surface and opposite the location of the forming acrosome. One of the centrioles (distal) gives rise to microtubules of the axoneme of the flagellum. The axoneme is surrounded by dense skeleton (coarse fibrils, rib-like fibrils and columns). The mitochondria aggregate in a helical manner around the proximal part of the flagellum axoneme, forming a thickened region known as the middle piece, the region where the movements of the spermatozoa are generated.

The reduced volume of the nucleus permits the sperm greater mobility and may protect the genome from the damage while it transit to the egg cell.

Residual cytoplasm is shed and excess cytoplasm is phagocytosed by Sertoli cells. Cytoplasmic bridges break down and the spermatozoa are released into the lumen of the seminiferous tubule.

The time required for a spermatogonium to become a mature spermatozoon is approximately 64 days.

During division of the spermatogonia, the resulting cells do not separate completely but remain attached by cytoplasmic bridges forming non-cellular structure called the syncitium. The intercellular bridges provide communication between every primary and secondary spermatocytes and spermatid derived from a single spermatogonium. By permitting the interchange of information from cell to cell, these bridges play an important role in coordinating the sequence of events in spermatogenesis. When the process of spermatogenesis is completed, the sloughing of the cytoplasm and cytoplasmic bridges as residual bodies leads to a separation of the spermatids.

The mature spermatozoon has a lash-like appearance and consists of a head, a neck, and a tail with middle, principal and terminal pieces (Fig.4). The head, which, for most mammals, has a flattened, oval shape, contains the condensed nucleus and is capped by an acrosome filled with hydrolytic enzymes. The midpiece contains large helical mitochondria that generate the energy. The tail contains microtubules.