Activin from secondary follicles causes small preantral follicles to remain dormant at the resting stage

The purpose of the present study was to investigate 1) whether activin A ca n cause primary follicles to become dormant at the resting stage, and 2) th e role of the secondary follicle on follicular growth of primary follicles. Preantral follicles (100-120 mu m in diameter) harvested from adult mice a nd cultured in in vitro follicle culture system showed a significant increa se in size and estrogen and inhibin secretion in response to FSK, but the a dministration of activin A blocked the effect of FSH. Withdrawal of activin A not only restored the follicular response to FSH but also enhanced the e ffect of FSH, indicating that the action of activin A is to cause small pre antral follicles to become dormant at the preantral stage. To investigate t he role of secondary follicles in early folliculogenesis, small preantral f ollicles were cocultured with secondary follicle (300-350 mu m in diameter) in the presence of FSH. The secondary follicle showed a significant increa se in follicular diameter as a result of stimulation by FSH, but the small preantral follicles did not increase in size. After removal of the secondar y follicle, however, the small preantral follicles commenced follicular gro wth, indicating that the growth of small preantral follicles is suppressed by the secondary follicle. Administration of the activin binding protein fo llistatin caused a significant increase in follicular diameter of both smal l preantral and secondary follicles as a result of stimulation by FSH. Thes e results have suggested that secondary follicles cause primary follicles t o become dormant at the resting stage by secreting activin.