2. Oogenesis
- It is the process of formation and maturation of ovum.
- It takes place in Ovarian follicles.
- Oogenesis is initiated in the embryonic stage when 2 million 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 prophase-I of meiosis and get temporarily arrested at that stage.
- Each primary oocyte gets surrounded by a layer of granulosa cells to form a primary follicle.
- Many 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 externa.
- The primary oocyte in the tertiary follicle grows and undergoes the first unequal meiotic division to form a large secondary oocyte (n) and a tiny first polar body (n). So, 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).
- The secondary oocyte forms a new membrane (zona pellucida).
- The Graafian follicle now ruptures to release the secondary oocyte (ovum) from the ovary. This is called ovulation.
Schematic Representation of Oogenesis
Structure of Ovum (Egg)
- Spherical and non-motile. About 0.2 mm in diameter.
- The ovum has 3 membranes:
- Plasma membrane: Innermost layer.
- Zona pellucida: Outer to the plasma membrane.
- Corona radiata: Outer layer formed of follicle cells.
Spermatogenesis & Oogenesis: A Comparison
| Spermatogenesis | Oogenesis |
|---|---|
| Occurs in testis. | Occurs in ovary. |
| Limited growth phase. | Elaborated growth phase. |
| Each primary spermatocyte gives 4 sperms. | Each primary oocyte gives one ovum. |
| No polar body formation. | Polar bodies are formed. |
| Begins at puberty and extends up to senility. | Begins at embryonic stage but suspends up to puberty. It ceases around the age of fifty. |