Question

Explain the steps in the formation of an ovum from an oogonium in humans.

Answer 1

1. Oogenesis is initiated in embryonic ovaries of the female fetus.
   
2. The cell starts division (oogonia) and enter into prophase 1 of the meiotic cycle and get temporarily arrested at that stage, called primary oocyte.
   
3. Each primary oocyte gets surrounded by a layer of granulosa cell and then called primary follicle. Many of these degenerate, hence by puberty only 60000 - 80000 primary follicle remain.
   
4. Further, it gets surrounded by more layers of granulosa and a new theca and form secondary follicle.
   
5. Soon these transform into the tertiary follicle, characterized by the presence of fluid-filled cavity called Antrum. At this stage, the primary oocyte grows and completes it's first meiotic division.
   
6. The division is unequal and leads to the formation of a large secondary oocyte and a tiny polar body.
   
7. The tertiary follicle further mature into Graafian follicle, the secondary oocyte forms a new layer called Zona pellucida surrounding itself.
   
8. During ovulation, this follicle ruptures to release secondary oocyte/ovum (which completes 2nd meiotic division just prior to fertilization)