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Gametes are sex cells of organisms, haploid in chromosome content that fuse with other haploid cells during fertilization in sexually reproducing organisms.
Purpose of Gamete Formation
Gametes or reproductive cells are always haploid in sexually reproducing organism in order to maintain the chromosomal number constant in the species after getting combined with the gamete from the other partner. The new organism has half the chromosomes from its mother and half from its father. Sexual reproduction also helps in the maintenance of genetic diversity of organisms because each organism has a different combination of genes than either of its parents. For example, in humans, a sperm cell gets fused with an egg cell, producing a zygote that has a unique set of genetic information and correct number of chromosomes.
Types of Gametes
There are two types of gametes in humans, male and female gametes. The male gamete is called sperm and has chromosome structure XY. The female gamete is called the ovum and has chromosome structure XX.
Spermatogenesis is a process of formation of male gametes that occurs in the seminiferous epithelium of the adult testis. The process of spermatogenesis gets completed in four phases and includes (1) the proliferation and differentiation of spermatogonia, (2) meiotic divisions of spermatocytes, and (3) the transformation of haploid round spermatids arising from the second meiotic division into spermatozoa(spermiogenesis), which (4) are released into the lumen of the seminiferous tubules (spermiation). Spermatogonia are the diploid (2n) stem cells of spermatogenesis and can be divided into type A and type B. Type A spermatogonia have an oval euchromatic nucleus in contrast to type B spermatogonia, which have a round heterochromatic nucleus (ref 1). It is the type B spermatogonia that get differentiated and enter the process of meiosis. Meiosis starts with DNA synthesis of type B spermatogonia which lose contact with the basal lamina (preleptotene). After completion of DNA synthesis, each chromosome consists of two chromatids (C). These cells are named primary spermatocytes, and the DNA content is tetraploid (2n;4C). Primary spermatocytes undergo the first meiotic division that results into the formation of haploid secondary spermatocytes with diploid DNA content (1n2 C). Secondary spermatocytes undergo the second meiotic division after a short interphase of about 6 h in the human without DNA synthesis. By this division, chromatids are finally separated leading to round spermatids with a haploid number of chromosomes and DNA content (1n1 C). The transformation of conventional round cell spermatids into spermatozoa with the capacity for motility and fertilization of an egg includes a complex sequence of events: (1) formation of the acrosome, (2) condensation of the nucleus, (3) development of the sperm tail, (4) reorganization of cellular organelles such as mitochondria and centrioles, and (5) reduction of the cytoplasm. The length of the human spermatozoon measures about 60 μm. The flat and oval head (diameter: 3 μm, length: 5 μm) consists of the acrosome and the extremely condensed nucleus. The acrosome covers the head surface and contains numerous proteolytic enzymes, i.e., hyaluronidase, collagenase, neuraminidase, phospholipase A, acrosin, and others. The release of these enzymes, the so-called acrosome reaction, enables the spermatozoon to penetrate the “corona radiata” of follicle cells and the zona pellucida of the egg. Some nuclear vacuoles are common. The flagellum measures about 55 μm in length. It possesses the central axoneme and is divided into neck piece, mid piece, principle piece, and end piece (Bergmann 2006; L'Hernault 2006; de Kretser et al. 1998).
Oogenesis occurs in the outermost layers of the ovaries. Oogenesis starts with a germ cell, called an oogonium (plural: oogonia). Oogonia undergo mitosis to increase in number, eventually resulting in up to one to two million cells in the embryo. The cells that undergo meiosis are called primary oocytes. These cells will begin the first meiotic division but get arrested during the first prophase stage. At the time of birth, all future eggs are in the prophase stage. At adolescence, anterior pituitary hormones cause the development of a number of follicles in an ovary (Gilbert 2000). This results in the primary oocyte finishing the first meiotic division. The cell divides unequally, with most of the cellular material and organelles going to one cell, called a secondary oocyte, and only one set of chromosomes and a small amount of cytoplasm going to the other cell. This second cell is called a polar body and usually dies. A secondary meiotic arrest occurs, this time at the metaphase II stage. At ovulation, this secondary oocyte will be released and travel toward the uterus through the oviduct. If the secondary oocyte is fertilized, the cell continues through the meiosis II, completing meiosis, producing a second polar body and a fertilized egg containing all 46 chromosomes of a human being, half of them coming from the sperm (Bukovsky et al. 2005; Virant-Klun 2015). In some species, such as sea urchins and frogs, the female routinely produces hundreds or thousands of eggs at a time, whereas in other species, such as humans and most mammals, only a few eggs are produced during the lifetime of an individual.In those species that produce thousands of ova, the oogonia are self-renewing stem cells that endure for the lifetime of the organism. In those species that produce fewer eggs, the oogonia divide to form a limited number of egg precursor cells. In the human embryo, the thousand or so oogonia divide rapidly from the second to the seventh month of gestation to form roughly seven million germ cells. After the seventh month of embryonic development, however, the number of germ cells drops precipitously (Gilbert 2000).
Gametogenesis is an essential process for the survival of the species. Any alteration in this process leads to high-risk pregnancy cases. Any disturbance or alteration in the process of spermatogenesis like oligospermia, azoospermia, asthenozoopermia, and chromosomal anomalies leads to the higher percentage of male infertility. On the other hand, in the case of females, various conditions like polycystic ovarian syndrome and premature ovarian failure contribute to female infertility cases around the globe. Various research studies are in process throughout the world for solving these issues of infertility. There are many herbal products in markets for humans after their preliminary study on animal models.
- Gilbert, S. F. (2000). Early development of the nematode Caenorhabditis elegans. In S. F. Gilbert (Ed.), Developmental biology (6th ed.). Sunderland: Sinauer Associates.Google Scholar
- Virant-Klun, I. (2015). Postnatal oogenesis in humans: A review of recent findings. Stem cells and cloning: advances and applications, 8, 49.Google Scholar