Gametogenesis is the process by which female and male sex cells or gametes, i.e., ova and sperms, are formed in the ovaries and testes respectively.
Two special types of cells called gametes take part in sexual reproduction. These cells are formed in gonads.
Male gonads or testes form male gametes or sperms and female gonads or ovaries form female gametes or ova.
Sperms were first discovered by Leeuwenhoek (1677) in human semen but ova were first identified as late as 1827 by Van Baer in mammals. Thus, “gametogenesis is that process of division in which gametes are formed in testes or ovaries.”
Before understanding gametogenesis, a knowledge of different types of cell divisions is necessary. Cell division is mainly of two types-mitotic and meiotic.
In mitotic cell division cach cell divides into daughter cells which are equal and similar in all respects. The number of chromosomes in daughter cells is the same as was in the parent cell.
Against this, in meiotic cell division, the process of division does not end after the formation of two daughter cells but takes place again.
In first division, half the chromosomes of the parent cell go in one daughter cell and half in the other, e.g., if parent cell had 24 chromosomes, then each daughter cell would have 12 chromosomes.
The meiotic division takes place only in gonads as a result of which gametes are formed. The number of chromosomes becomes half in gametes. This division is also known as reduction division.
Reduction of the number of chromosomes to half is very essential in gametes because we know that the zygote is formed by the union of male and female gametes in sexual reproduction.
Thus, the number of chromosomes becomes normal in zygote. If reduction division does not take place, the number of chromosomes would become double in zygote.
Since every species has a fixed number of chromosomes it is necessary that the number of chromosomes in gametes is half that of the parents.
The process of formation of male gametes or sperm cells from male germ cells is known as spermatogenesis and that of the formation of female gametes or ova from female germ cells is called oogenesis.
All metazoans, which reproduce sexually begin their life as a single cell called zygote or fertilized egg or ovum.
This cell is the product of union of two cells-the male and female sex cells or gametes. The fertilized egg through successive division and differentiation develops into a young animal.
So the gametes are involved in sexual reproduction. They are of two types, the male gametes or spermatozoa and female gametes or ova.
The spermatozoa (sing. spermatozoon) are formed inside the testis of the male and ova (sing. ovum) are formed inside the ovary of female.
These gametes are highly specialized cells which take part in the process of fertilization. The gametes are formed from the germinal epithelial cells which line the testis and ovary.
Therefore, gametogenesis can be defined as “the process by which gametes are produced from the germinal epithelial cells of the gonads.”
In the process of gametogenesis, the basic principle is that the chromosome number is reduced to half (n) in the male as well as female gametes.
This reduced number is called haploid and after fertilization, the normal diploid number of chromosomes (2n) is restored by the fusion of male and female gametes. In this process mitotic and meiotic cell divisions take place.
Accordingly the gametogenesis is of two types: (A) Spermatogenesis and (B) Oogenesis. Spermatogenesis deals with the origin, development and maturation of male sex cells or spermatozoa. But oogenesis deals with the origin, development and maturation of female sex cells or ova.
Both the processes (spermatogenesis and oogenesis) pass through three phases namely:
- Phase of multiplication.
- Phase of growth.
- Phase of maturation.
Types of Gametogenesis
Gametogenesis is the process by which gametes are produced from the germinal epithelium the gonads.
It is of two types: (1) spermatogenesis and (2) oogenesis.
Spermatogenesis deals with the origin, development and maturation of male gametes or spermatozoa, whereas oogenesis is concerned with the origin, development and maturation of female gametes or ova.
Both of these processes occur of in three phases, namely: (a) phase of multiplication (b) phase of growth and (c) phase of maturation.
Spermatogenesis involves production of spermatids followed by their metamorphosis called spermiogenesis or spermatoleosis.
During the phase of multiplication, the primordial germ cells lining the seminiferous tubules of the testis divide repeatedly to produce a large number of cell called spermatogonia or sperm mother cells, containing diploid number of chromosomes (2n).
During growth phase, the spermatogonia grow to form primary spermatocytes (2n).
In the maturation phase, each primary spermatocyte undergoes meiosis forming four spermatids (n). These spermatids undergo further changes and are converted to spermatozoa.
Know more about spermatogenesis in detail.
During oogenesis, the primordial germ cells of the ovary divide repeatedly to produce oogonia or egg mother cells (2n).
In the growth phase, these cells increase greatly in size due to accumulation of yolk and other nutritive substances.
The nucleus also becomes very large with lamp brush chromosomes. After these changes, the oogonial cells become known as primary oocytes.
During maturation phase, this oocyte undergoes meiosis producing one large ovum or egg and three polar bodies, all with haploid number of chromosomes.
The polar bodies soon disintegrate. Animal eggs are classified according to the amount and distribution of yolk inside the cytoplasm.
Know more about Oogenesis in detail.
Significance of Gametogenesis
Gametogenesis involves formation of sperms during spermatogenesis and ovum during oogenesis. During both the processes the chromosome number is reduced to half resulting sperms and ova.
From one spermatocytes four sperms are formed, whereas a single ovum is produced during oogenesis.
The reduction in chromosome number in sperm (n) and ovum (n) helps in restoring the paternal chromosome number (2n) during fertilization.