Figure 1.

Hormonal regulation of preantral follicle growth. Selective primordial follicles develop to the primary stage under the control of AKT and mTOR signaling (initial recruitment), whereas most primordial follicles remain arrested by dormancy factors. Once initiated to growth, primordial follicles develop through primary and secondary stages before acquiring an antral cavity. Although most early antral follicles undergo atresia, select antral follicles supported by cyclic changes in pituitary FSH and LH reach the preovulatory stage, capable of releasing mature oocytes after ovulation for fertilization (cyclic recruitment) (1). In addition to the well-studied role of FSH on antral follicle growth (above the dashed line), FSH also regulates preantral follicle growth, together with a large number of oocyte- and granulosa cell-derived paracrine factors (below the dashed line). Furthermore, development of preantral and antral follicle is restrained by the inhibitory Hippo signaling pathway.