Oocyte development in bovine primordial follicles is promoted by activin and FSH within a two-step serum-free culture system

Quiescent follicles of large mammals initiate growth within cultured pieces of ovarian cortex. Systems capable of sustaining in vitro development from this early stage until oocyte maturation would allow investigation of mechanisms regulating oocyte development in its entirety. The aims of this study were 1) to determine whether bovine follicles initiated to grow in vitro could be isolated from the cortical environment, and could undergo further development and 2) to evaluate the effect of activin and FSH on the development of secondary follicles derived from primordial follicles. Fragments of bovine ovarian cortex were cultured in serum-free medium for 6 days; thereafter, secondary follicles were isolated for further culture. After a maximum total of 21 days in vitro, follicles were either processed for histological assessment or opened to release the oocyte-cumulus complexes for inspection by light microscopy. Compared with control, significant follicle and oocyte growth were observed in activin-exposed follicles, with or without FSH, with some oocyte diameters measuring over 100 microns following a total in vitro period of 15 days. Significant oestradiol secretion was observed in follicles cultured in activin alone after a total of 9 days in vitro compared with other treatment groups; however, this effect was not sustained. In summary, this study demonstrates the promotion of primordial bovine follicle development within a two-step serum-free culture system with oocyte diameters >100 mu m achieved over 15 days in vitro. Further development of this system is needed to support complete oocyte growth and thereafter in vitro maturation.