- It is the process of formation and maturation of ovum.
- It takes place in Ovarian follicles.
- Oogenesis is initiated in embryonic stage when 2 million 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 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 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 tertiary follicle grows and undergoes first unequal meiotic division to form a large secondary oocyte (n) & a tiny first polar body (n). So, secondary oocyte retains nutrient rich cytoplasm of primary oocyte.
- It is unknown that 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
Structure of ovum (egg)
- Spherical and non-motile. About 0.2 mm in diameter.
- Ovum has 3 membranes:
a. Plasma membrane: Innermost layer.
b. Zona pellucida: Outer to the plasma membrane.
c. Corona radiata: Outer layer formed of follicle cells.
Spermatogenesis & Oogenesis- A comparison
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.