Relationship between in vitro growth of bovine oocytes and steroidogenesis of granulosa cells cultured in medium supplemented with bone morphogenetic protein-4 and follicle stimulating hormone

Bone morphogenetic protein-4 (BMP-4) and FSH play important regulatory roles in follicular growth and steroidogenesis in vivo. The purpose of this study was to investigate the effects of BMP-4 and FSH on in vitro growth (IVG) and steroidogenesis of bovine oocyte-cumulus-granulosa complexes (OCGCs). We cultured OCGCs collected from early antral follicles (0.5-1 mm) in medium without BMP-4 and FSH for 4 days and investigated the appearance of OCGCs and their steroidogenesis. During the first 4 days of IVG, morphologically normal OCGCs produced more estradiol-17β (E2), but less progesterone (P4). Morphologically normal OCGCs were subjected to an additional culture in medium supplemented with BMP-4 (0, 10, and 50 ng/mL) and FSH (0 and 0.5 ng/mL) until day 12. We examined the viability and steroidogenesis of OCGCs after 8 and 12 days of culture. Oocyte growth, characteristics of granulosa cells, and the maturational competence of oocytes were also investigated. On day 8, the viability of OCGCs cultured without FSH was higher in the 10 ng/mL BMP-4 group than in the 50 ng/mL BMP-4 group (P < 0.05). No significant difference was observed in the viability of groups cultured with FSH, regardless of the addition of BMP-4, and FSH improved the viability of 50 ng/mL BMP-4 group similar to 10 ng/mL BMP-4 group. The total number of granulosa cells was larger in 10 ng/mL BMP-4 group cultured with FSH than in 50 ng/mL BMP-4 group cultured with FSH on day 8 (P < 0.05). E2 production decreased from days 8-12, and P4 production increased throughout IVG culture, regardless of the addition of BMP-4 and FSH (P < 0.05). No significant differences in E2 production were observed between groups from days 4-8, regardless of whether BMP-4 was added without FSH; however, E2 production in the group cultured with 50 ng/mL BMP-4 was suppressed by FSH. BMP-4 suppressed E2 production from days 8-12, regardless of whether FSH was added. The group cultured with 10 ng/mL BMP-4 without FSH showed the lowest P4 production among all groups for all culture periods. OCGCs that produced mature oocytes tended to secrete more E2 and less P4 than OCGCs that produced immature oocytes. In conclusion, until day 8 of the IVG culture, P4 production by OCGCs was suppressed by the addition of 10 ng/mL BMP-4 in the absence of FSH, without inhibiting E2 production. These conditions appear to mimic growing follicles until day 8 and mimic degenerating follicles from days 8-12 of culture.