Gametogenesis
Gametogenesis is the biological process by which male and female sex cell or gametes, i.e. sperm and ova are formed respectively in the male and female gonads (testes and ovaries).
Gametogenesis is the process of division of diploid cell (n) to produce new haploid cell (2n). In humans, two different types of gametes are present. Male gametes called sperm and female gametes called the ovum. The gametes differ from all other cell (=somatic cell) of the body as their nuclei contain only half the number of chromosome found in the nuclei of somatic cell.
Types of Gametogenesis
Spermatogenesis and oogenesis are both forms of gametogenesis, in which a diploid gametes cell produced haploid sperm and egg cells.
- Spermatogenesis: Formation of sperm
- Oogenesis: Formation of ovum
Spermatogenesis
In male reproductive system, spermatogenesis is the biological process of the production of sperms from the immature male germ cell. Sperm are form in the seminiferous tubules of testes, the male reproductive organ. spermatogenesis starts at puberty and usually continues uninterrupted until death, although a slight decrease in the quantity of sperm is seen with the increase in age.
Phase of spermatogenesis
There are three phases of spermatogenesis are:
- Multiplication phase
- Growth phase
- Maturation phase
Multiplication phase
This phase is also know as proliferation and renewal of spermatogonia. During this phase the diploid spermatogonia which are situated at the periphery of the seminiferous tubule, multiply mitotically to form spermatocytes and also to give rise to new spermatogonia stem cells and enter the phase of growth.
Growth phase
During this phase, a limited growth of spermatogonia taken place; their volume becomes double and they are now called primary which spermatocytes which are still diploid in number. Now these primary spermatocytes enter into the next phase namely, maturation phase.
Maturation phase
The maturation phase is the final phase. Primary spermatocytes undergo meiosis. Meiosis-1 being reductional division forms two haploid secondary spermatocyte cells. Meiosis-2 being equational division, form four haploid spermatids. Spermatids change into motile sperm by changes in structure.
Oogenesis
In female reproductive system, oogenesis is the biological process of the formation of eggs within ovary. It is developed from the primary oocyte by maturation. Oogenesis is initiated in the embryonic stage. In the female ovary, millions of oogonia or mother cells are formed during fetal development.
Phases of Oogenesis
There are three phases of oogenesis are:
- Multiplication phase
- Growth phase
- Maturation phase
Multiplication phase
The primary germinal cells of the ovary with diploid number chromosome (2n) divided several times mitotically so a to form a large no. of daughter cell know as oogenesis.
Growth phase
The growth phase of the primary oocyte is very long. It varies from a few days to many years. Growth phase takes about 6-14 days in hen after ovulation, but 3 year in frog and in women all the oocytes are present at the time of birth but no one grows till the attainment of puberty, i.e. 12-14 years. However afterwards they grow one by one.
During the growth phase, the following changes occur:
- The primary oocyte increases many folds. The size increases due to the accumulation of reserve food like protein and fat in the form of yolk. Due to heavy weight, it is usually concentrated toward the lower portion of the egg forming the vegetative pole. The portion of the cytoplasm with egg pro-nucleus remains often separated from the yolk and occurs toward the upper portion of the egg forming the animal pole.
- Number of mitochondria increase and in certain cases they are concentrated in the same place to form mitochondrial clouds.
- Increase the amount of endoplasmic reticulum and activity of the Golgi complex.
- Formation of thin vitelline membrane around the oocyte.
- Due to increase in the amount of DNA, nuclear sap and number of nucleoli, nucleus increases in size.
- In the growth phase nucleolar genes which code for ribosomal RNA and are located in the nucleolar organize region multiply to facilitate the rapid synthesis of ribosomal RNA. This multiplication of genes without mitosis is know as gene amplification or redundancy.
Maturation phase
In the maturation phase, the nucleus of the oocyte undergoes two maturation divisions. The first division is meiotic; as a result two haploid (n)cells are produced. In this division, cytokinesis is unequal; the large daughter cell with almost entire cytoplasm and yolk forms the secondary oocyte. while the smaller one with the haploid nucleus (n) and almost without cytoplasm forms the first polar body which is given off from the surface of oocyte at the animal pole. The secondary oocyte with the haploid number of chromosomes undergoes second maturation division or second meiotic division. The division is also unequal the large one containing yolk is called ovum and small cell is the second polar body. The first polar may also divide thus, producing the total of three polar bodies which degenerate soon. So, as a result of oogenesis, only one functional ovum is formed from a primary oocyte. In most of the vertebrates, the first meiotic division occurs with the commencement of the growth phase, and secondary maturation division occurs when the egg is activated by the entry of sperm.