Fig. 12. The possible effect of neurotrophins on folliculogenesis. Oogonia and pregranulosa cells associate into primordial follicles, with flattened granulosa cells, (a `resting' stage of development). Oocytes with too few granulosa cells die at this point. Surviving oocytes are contained in follicles that have enough granulosa cells to provide the oocyte with sufficient neurotrophins. Follicles that leave the primordial stage and enter the primary stage exhibit rounding up of granulosa cells and undergo oocyte growth. Trk receptors have no effect on primary follicle survival.