Primordial follicle activation: to be or not to be takes another twist

In mice, following the proliferation of primordial germ cells, oogonia enter meiosis circa embryonic day 13 and are then referred to as oocytes in the embryonic mouse ovary. Oocytes in the embryonic ovary exist in clusters, surrounded by epithelial pregranulosa cells [1]. Germ cell cyst breakdown results in the envelopment of individual oocytes by flat granulosa cells to form primordial follicles. Primordial follicles contain small oocytes (<20 μm in mice) surrounded by flat granulosa cells. These early steps in the formation of primordial follicles are critical since primordial follicles constitute the fundamental reproductive units of the ovary and give rise to all dominant follicles. Primordial follicle growth activation (PFGA) is a process by which primordial follicles are selected into the growing follicle pool [2]. The PFGA is thought to be a critical step in determining reproductive life span, predisposition to infertility and primary ovarian insufficiency, as...