1.2 Folliculogenesis1.2.1 Formation of folliclesIn mice, the journey of oocyte begin from the development of primordial germ cells (PGCs) which initially become visible at embryonic day 7.

5 (E7.5) in the extra-embryonic mesoderm. These PGCs travel via hindgut and dorsal mesentery to settle at the genital ridge at E10.5. Further, the PGCs experience numerous mitotic divisions and increase in number all the way through their migration (Ginsburg et al., 1990).

The dividing PGCs gets attached with each other through intercellular bridges because of incomplete cytokinesis which leads the formation of numerous germ cell aggregates or oogonia inside the female gonad (Pepling and Spradling, 1998). By E13.5, meiosis commences in the oogonia and at this stage they are referred as oocytes (McLaren and Southee,1997; McLaren, 2000). Oocytes inside aggreagates starts passing through the prophase of meiosis I and gets arrested in the diplotene stage at about E17.5 (Borum, 1967).

At this stage, few number of somatic cells also surround the germ cell cysts and these somatic cells are termed as pregranulosa cells.