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Gametogenesis
- 1. 1. Gametogenesis (Spermatogenesis and Oogenesis) Gametogenesis is a biological process by which diploid or haploid precursor cells undergo cell division and differentiation to form mature haploid gametes. The existence of a multicellular, haploid phase in the life cycle between meiosis and gametogenesis is also referred to as alternation of generations. Gametogenesis is the process by which male and female sex cells or gametes, i.e., sperms and ova are formed respectively in the male and female gonads (testes and ovaries). Gametogenesis for the formation of sperms is termed spermatogenesis, while that of ova is called oogenesis. Both spermatogenesis and oogenesis comprise similar phases of sequential changes viz., (i) multiplication phase, (ii) growth phase and (iii) maturation phase. Spermatogenesis: The process of formation of sperms is called spermatogenesis. It occurs in the seminiferous tubules of the testes. The seminiferous tubules are lined by germinal epithelium. The germinal epithelium consists largely of cuboidal primordial germ cells. Spermatogenesis includes formation of spermatids and formation of spermatozoa.
- 2. (i) Formation of Spermatids: It includes the following phases. (a) Multiplication Phase: At sexual maturity, the undifferentiated primordial germ cells divide several times by mitosis to produce a large number of spermatogonia. (b) Growth Phase: Spermatogonium actively grows to a larger primary spermatocyte by obtaining nourishment from the nursing cells. (c) Maturation Phase: Each primary spermatocyte undergoes two successive divisions, called maturation divisions. The first maturation division is reductional or meiotic. Hence, the primary spermatocyte divides into two haploid daughter cells called secondary spermatocytes. Which is an ordinary mitotic division to form, four haploid spermatids, by each primary spermatocyte. (ii) Spermatogenesis: The transformation of spermatids into spermatozoa is called spermiogenesis. The spermatozoa are later on known as sperms. Hormonal Control of Spermatogenesis: Spermatogenesis is initiated due to increase in gonadotropin-releasing hormone (GnRH). GnRH acts on the pituitary gland to secrete luteinizing hormone (LH) and follicle stimulating hormone (FSH). LH acts on the Leydig’s cells of the testes to secrete testosterone. FSH acts on spermatogonia to stimulate sperm production. Significance of Spermatogenesis: (i) During spermatogenesis, one spermatogonium produces four sperms, (ii) Sperms have half the number of chromosomes. After fertilization, the diploid chromosome number is restored in the zygote. It maintains the chromosome number of the species, (iii) During meiosis I crossing
- 3. over takes place which brings about variation, (iv) Spermatogenesis occurs in various organisms. Thus it supports the evidence of the basic relationship of the organisms. Spermatozoon The sperms are microscopic and motile cells. Sperms remain alive and retain their ability to fertilize an ovum (egg) from 24 to 48 hours after having been released in the female genital tract. A typical mammalian sperm consists of a head, neck, middle piece and tail. (i) Head: It contains anterior small acrosome and posterior large nucleus. Acrosome is formed from Golgi body of the spermatid. Acrosome contains spermlysins enzymes that are used to contact and penetrate the egg (ovum) at the time of fertilization. ii) Middle piece: The middle piece contains the mitochondria coiled round the axial filament called mitochondrial spiral. They provide energy for the movement of the sperm. (iii) Tail: The tail is several times longer than the head. The sperm swims about by its tail in a fluid medium. Oogenesis The process of formation of a mature female gamete (ovum) is called oogenesis. It occurs in the ovaries (female gonads). It consists of three phases: multiplication, growth and maturation. (a) Multiplication phase: These cells divide by mitosis, producing a couple of million egg mother cells or oogonia in each ovary of the foetus. The oogonia multiply by mitotic divisions forming the primary oocytes. (b) Growth phase: This phase of the primary oocyte is very long. It may extend over many years. The oogonium grows into a large primary oocytes. Each primary oocyte then gets surrounded by a layer of granulosa cells to form primary follicle. A large number of these follicles degenerate during the
- 4. period from birth to puberty. So at puberty only 60,000- 80,000 primary follicles are left in each ovary. (c) Maturation phase: Each primary oocyte undergoes two maturation divisions, first meiotic and the second meiotic. In the first, meiotic division, the primary oocyte divides into two very unequal haploid daughter cells— a large secondary oocyte and a very small first polar body. In the second maturation division, the first polar body may divide to form two second polar bodies. The secondary oocyte again divides into unequal daughter cells, a large ootid and a very small second polar body. Thus from one oogonium, one ovum and three polar bodies are formed. The ovum, is the actual female gamete. The polar bodies take no part in reproduction and, hence, soon degenerate. In humans, the first polar body does not undergo meiosis II, whereas the secondary oocyte proceeds as far as the metaphase stage of meiosis II. However, it then stops advancing any further; it awaits the arrival of sperm for completion of meiosis II. Completion of meiosis II converts the secondary oocyte into a fertilized ovum (egg) or zygote (and also a second polar body). Hormonal Control of Oogenesis:
- 5. GnRH secreted by the hypothalamus stimulates the anterior lobe of pituitary gland to secrete LH and FSH. FSH stimulates the growth of Graafian follicles and also the development of egg/oocyte within the follicle to complete the meiosis I to form secondary oocyte. FSH also stimulates the formation of oestrogens. LH induces the rupture of the mature Graafian follicle and thereby the release of secondary oocyte. The remaining part of the Graafian follicle is stimulated by LH to develop into corpus luteum (“yellow body”). The rising level of progesterone inhibits the release of GnRH, which in turn, inhibits production of FSH, LH and progesterone. Significance of Oogenesis: (i) One oogonium produces one ovum and three polar bodies. (ii) Polar bodies have small amount of cytoplasm. It helps to retain sufficient amount of cytoplasm in the ovum which is essential for the development of early embryo. (iii) During meiosis first crossing over takes place which brings about variation. (iv) Oogenesis occurs in various organisms. Therefore, it supports the evidence of basic relationship of the organisms. Gametogenesis is important because it is the process that produces your functional gametes (egg and sperm cell) either through meiosis from a diploid cell or mitosis from a haploid cell (plants). These gametes are important because this allows geneticaly-varied organisms (offspring) to be formed. Dr. K. Rama Rao