Oogenesis is the process of formation of mature female gametes or ova. These cells start division and enter prophase I of meiosis and remain suspended at that stage; these are called primary oocyte.
Each primary oocyte is surrounded by a layer of granulosa cells and become the primary follicle. When the primary follicle becomes surrounded by more layers of granulosa cells, it is called secondary follicle. The secondary follicle transforms into tertiary follicle. With the development of a fluid filled cavity (antrum) around the primary oocyte.
The granulosa cell become organised into an outer layer, called theca external and an internal interna. At this stage, the primary oocyte completes meiosis I and form a larger haploid secondary oocyte and a tiny polar body.
A tertiary follicle grow further and changes into mature follicle or agrarian follicle. The secondary oocyte secretes a new membrane called zona pellucida around it. At this stage the follicle ruptures to release the secondary oocyte, which moves into the fallopian tube. The secondary oocyte completes meiosis II only when a sperm enters its cytoplasm, it form a larger cell and Ootid and a smaller cell, the second polar body.
