BMP-15 main functions are crucial for the beginning of folliculogenesis as seen in Image 1. The primordial follicle is made up of the oocyte and a single layer of flattened granulosa cells. BMP-15 is released from the oocyte into the surrounding granulosa tissue where it binds to two membrane bound receptors on granulosa cells. This promotes granulosa cell proliferation via mitosis. BMP-15  promotes the change of primordial to primary and secondary follicles which are surrounded by several granulosa cell layers but doesn't promote transition into preovulatory follicles. -15 prevents differentiation into preovulatory follicle by inhibiting FSH action in granulosa. FSH is released by the anterior pituitary as part of the hypothalamic-pituitary-gonadal axis and promotes the differentiation of early follicles into later preovulatory ones. BMP-15 prevents this transition by inhibiting the production of FSH receptor mRNA in granulosa cells. Therefore, FSH cannot bind to the granulosa cells, this inhibits FSH dependent progesterone production and luteinization, subsequently granulosa cells do not differentiate.