“Gametogenesis” is the process of gamete formation in the gonads (testis and ovary) in sexually reproducing animals.
The two processes of gametogenesis are:
(i) Spermatogenesis:
Formation of haploid spermatozoa (sperms) from diploid spermatogonia.
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(ii) Oogenesis:
Formation of haploid ovum from the diploid germ cells in the ovary.
(i) Spermatogenesis:
The process of formation of spermatozoa (sperms) from diploid spermatogonia is called spermatogenesis.
It includes the following phases:
(a) Multiplication phase:
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The male germ cells (spermatogonia) present on the inside wall of seminiferous tubules multiply by mitotic division and increase in numbers.
(b) Growth phase:
Spermatogonia grow and increase in size and forms primary spermatocytes. Each spermatogonia is diploid and contains 46 chromosomes.
(c) Maturation phase or formation of spermatids:
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Some of the spermatogonia called primary spermatocytes periodically undergo meiosis. A primary spermatocyte completes the first meiotic division (reduction division) leading to formation of two equal haploid cells called secondary spermatocytes, which have only 23 chromosomes each. The secondary spermatocytes undergo the second meiotic division to produce four equal haploid spermatids.
(d) Differentiation phase:
The spermatids are transformed into spermatozoa (sperms) by the process of spermiogenesis. The sperm’s head gets attached to Sertoli cells to draw nourishment and are finally released from the seminiferous tubules by the process called spermiation.
Hormonal control of spermatogenesis:
(i) Spermatogenesis is initiated at the age of puberty by the gonadotropin releasing hormone (GnRH). GnRH is secreted by the hypothalamus.
(ii) The increased levels of GnRH stimulate the anterior pituitary which then secretes the FSH (follicle stimulating hormone) and LH (luteinising hormone). FSH stimulates Sertoli cells to secrete some factors which help in spermiogenesis.
(iii) LH acts on the Leydig cells and causes the secretion of testosterone from the testes.
Structure of a sperm (Spermatozoa):
1. It consists of four parts-head, neck, middle piece and tail-enveloped by a plasma membrane.
2. Head:
It is the enlarged end of a sperm, containing the large haploid nucleus, i.e., condensed chromatin body and is capped by acrosome. The acrosome contains hydrolytic enzymes that help in dissolving membranes of the ovum for fertilisation.
3. Neck:
It contains proximal centriole towards the nucleus which is necessary for the first cleavage division of zygote and the distal centriole that is connected to the tail filament.
4. Middle piece:
It contains a number of mitochondria that provide energy for the movement of the tail that facilitate sperm motility essential for fertilisation.
5. Tail:
It consists of axial filaments surrounded by the plasma membrane. It helps to swim the sperms in a fluid medium.
6. A human male ejaculates about 200-300 million sperms during a coitus. Seminal plasma along with the sperms constitute the semen.
(ii) Oogenesis:
The process of formation of a mature female gamete is called oogenesis. It occurs in the ovaries. It consists of the following three phases:
1. Multiplication phase:
Genesis is initiated during the embryonic development stage when a couple of million gamete mother cells (oogonia) are formed within each foetal ovary. No more oogonia are formed and added after birth.
These cells start division and enter into prophase-I of the meiotic division. They get temporarily arrested at this stage and are called primary oocytes.
2. Growth phase:
Each primary oocyte then gets surrounded by a layer of granulosa cells. This structure is called the primary follicle. A large number of these follicles degenerate during the phase from birth to puberty.
At puberty, only 60,000 to 80,000 primary follicles are left in each ovary. The primary follicles get surrounded by more layers of granulosa cells and a new theca to form secondary follicles.
3. Maturation phase:
In the first maturation phase, the secondary follicle soon transforms into a tertiary follicle. The primary oocyte within the tertiary follicle grows in size and completes its first meiotic division to form a large haploid secondary oocyte and a tiny first polar body.
The tertiary follicle changes into a mature follicle-the Graafian follicle-which ruptures to release the secondary oocyte (ovum) from the ovary by a process called ovulation.
The second maturation phase occurs after fertilisation when the meiotic division of the secondary oocyte is complete. This second meiotic division results in the formation of a second polar body and a haploid ovum (ootid).