The formation of egg (female gamete) in ovary (gonads) is knwon as Oogenesis. Just like testis ovary also contain germinal epithelium. The germinal epithelial cells are small diploid cells. These cells undergo a number of stages to produce egg.
Important stages of oogenesis
There are three main stages of oogenesis
1. Multiplication phase: The primordial germinal cells divide repeatedly to form the oogonia. This initial steps occur prior to birth. By the time the foetus is 25 weeks old, all the oogonia that she will ever produce, are already formed by mitosis. No more oogonia are formed and added after birth.
2. Growth phase: Only some of the oogonia increase in size and undergo a growth phase to form the primary oocytes. Oogonia start division and enter into prophase-I of the meiotic division and get temporarily arrested at that stage, called primary oocytes. Each primary oocytes then gets surrounded by a layer of granulosa cells and is called the primary follicle. A large number of these follicles degenerate during the phase from birth to puberty. Therefore, at puberty only 60,000-80,000 primary follicles are left in each ovary. Primary oocytes are diploid in nature. The growth phase of the oogenesis is comparatively longer than the growth phase of spermatogenesis. In the growth phase, the size of the primary oocytes increase enormously.
3. Maturation phase: The primary oocytes undergoes the first meiotic division to produce two haploid cells. Since there is an unequal division of the cytoplasm, one of the cells is large and receives most of the cytoplasm (secondary oocytes) and other is small (Secondary oocytes). Ovulation occurs in secondary oocyte stage. The second part of the meiosis take place only after the contact of sperm. The nucleus of secondary oocytes divides again to form another polar body and Ootid. As there is no metamorphosis in Ootid, it may be called as Ovum.
Structure of Ovum
Ovum is the immotile female reproductive cell. It is produced in the ovary. A single ovum is released from the ovary at regular intervals. Mammalian eggs have very less amount of yolk, so the eggs are oligolecithal and isolecithal or microlecithal. The egg has 2 egg membranes, Zona pellucida and Corona radiata.
- Zona pellucida: This is a transparent membrane like covering and is a primary membrane secreted by the ovum/oocyte itself.
- Corona radiata: This is a layer of follicular cells. These cells are attached to the surface of egg through "hyaluronic acid". This is a secondary membrane, which is secreted by the ovary. These eggs don't have tertiary membrane.