Oogenesis:

1. The phenomenon of formation of haploid ovum (egg) from the diploid egg mother cell of ovary (female gonad) is called oogenesis.

2. Oogenesis is initiated during embryonic development in a female foetus. It occurs in three phases i.e. multiplication, growth and maturation.

3. Multiplication. Some cells of the germinal epithelium of ovary function as primary germ cells and are called oogonia. All the oogonia (about a million) are formed in the ovary of the foetus of 25 weeks. No new oogonia are formed after birth.

4. The oogonial cells which enter meiosis I are the primary oocytes but the division remains suspended in prophase I. Other cells form the granulosa/ follicle cells and forego the potential of forming ova.

5. Growth phase. This period of growth of oogenesis is very prolonged one and the increase in size of primary oocyte is very considerable. It is a slow process and may take a long period.

6. The primary oocyte becomes surrounded by a layer of granulosa cells and is called the primary follicle.

7. A large number of these follicles degenerate before puberty (follicular atresia) and only about 60000-80000 are left in the ovary.

8. The primary follicles become surrounded by more granulosa cells and a thecal layer, at this stage they are called secondary follicles.

9. As development continues, a cavity develops in the secondary follicle around the primary oocyte; it is called antrum and is filled with a fluid called liquor folliculi.

10. Simultaneously, the thecal layer becomes organized into an outer theca externa and at this stage the structure is called tertiary follicle.

11. In human female the oogonia are already formed at the time of birth and show growth only after attaining sexual maturity.

12. Maturation phase. Now the primary oocyte undergoes first meiotic division and gives rise to two daughter cells, larger called secondary oocyte and smaller as first polar body. The secondary oocyte divides again and gives rise to haploid ootid or the ovum and the 2nd polar body.

13. Size of the egg depends upon the amount of reserve food material.

14. Thus one ovum (X) is formed from one primary oocyte (2X) as a result of oogenesis.

15. The oogenesis is also controlled by FSH hormone.