What are the changes in the oogonia during the transition of a primary follicle to Graafian follicle ?

The cells of germinal epithelium divides rapidly and forms many diploid oogonia. Oogonia grows and forms primary oocytes.

Each primary oocytes then gets surrounded by a layer of granulosa cells and then called primary follicle.

The primary follicle gets surrounded by more layer granulosa cell and a new theca called secondary follicle. The secondary follicle soon transforms into a tertiary follicle which is characterised by a fluid filled cavity called antrum.

The primary oocyte within the tertiary follicle grows in size and that completes its first meiotic division. It is an unequal division resulting in the formation of large haploid secondary oocyte and a tiny first polar body. The tertiary follicle further changes into mature follicle or graafian follicle which breaks and releases secondary oocyte from the ovary. This is called ovulation.