Oogenesis
- It is the process of formation and maturation of ovum.
- It takes place in Graafian follicles.
- Oogenesis is initiated in embryonic stage when millions of egg mother cells (oogonia) are formed within each ovary.
- No more oogonia are formed and added after birth.
- Oogonia multiply to form primary oocytes. They enter into prophase-I of the meiosis and get temporarily arrested at that stage.
- Each primary oocyte gets surrounded by a layer of granulosa cells to form a primary follicle.
- A large number of primary 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 follicles get surrounded by more layers of granulosa cells and a new theca to form secondary follicles.
- The secondary follicles transform into a tertiary follicle. It has a fluid filled cavity (antrum). The theca layer forms an inner theca interna and an outer theca external.
- The primary oocyte within the tertiary follicle grows in size and undergoes first unequal meiotic division to form a large haploid secondary oocyte and a tiny first polar body. The secondary oocyte retains the nutrient-rich cytoplasm of the primary oocyte.
- It is unknown whether the first polar body divides further or degenerates.
- The tertiary follicle further changes into the mature follicle (Graafian follicle).
- Secondary oocyte forms a new membrane (zona pellucida).
- Graafian follicle now ruptures to release the secondary oocyte (ovum) from the ovary. This is called ovulation.
Schematic representation of oogenesis
Oogonia (egg mother cells)-2n (46 chromosomes)
↓ Mitosis differentiation
Primary oocyte- 2n (grow in size)
↓ 1st meiotic division (prior to ovulation)
Secondary oocyte (n) & first polar body (n)
↓ 2nd meiotic division (during fertilization)
Ovum (n) & second Polar body (n)
