Improvement on in vitro maturation, fertilization and development of minke whale (Balaenoptera acutorostrata) oocytes

The aims of the present study were to improve in vitro maturation, fertilization and subsequent development of minke whale oocytes. We investigated the effects of different concentrations (0, 10 and 20%) of fetal whale serum (FWS) in maturation medium on nuclear maturation, morphological grade (A or B) of cumulus-oocyte complexes (COC) obtained from prepubertal and adult minke whales. Grade A (> or = 5 layers of cumulus cells) COC collected from the adult whales and cultured in the medium with 20% FWS had a higher (P < 0.05) maturation rate (31.8%) than those in the medium without FWS (0%). Adding FWS to the maturation medium significantly (P < 0.01) improved the proportion of oocytes at Metaphase II (M-II): without FWS (7.9%), with 10% (19.4%) and 20% (21.4%) FWS. However, sexual maturity of whales and COC grades were not significantly affected by M-II oocytes. When in vitro fertilization of matured oocytes was performed in the presence of 20% FWS or 0.6% BSA in the fertilization medium, the proportions of sperm penetration and two-pronuclei formation in matured oocytes were not significantly different. Grade A COC cultured in a culture medium supplemented with 10% FWS cleaved at a higher rate (15.4%, P < 0.05) than did Grade A and B COCs cultured in the medium without FWS (0%). Neither Grade A nor B COCs cleaved when the medium was without FWS. The proportions of cleaved oocytes increased (P < 0.05) with FWS supplementation (6.9% and 8.1% for 1.0% FWS and 20% FWS, respectively). Grade A COC was significantly (P < 0.05) superior in its ability to cleave (14.5%) and develop to morula (4.2%) compared with that of the oocytes from Grade B COC (2.5% and 0%). Coculture with granulosa cells during in vitro culture did not significantly affect cleavage and development to the morula stage. These results indicate that FWS addition in the maturation medium improved the rate of in vitro maturation and cleavage after insemination of minke whale oocytes. The BSA supplementation in fertilization medium was as effective as FWS supplementation for in vitro fertilization of matured oocytes. In vitro embryo production beyond the morula stage of minke whale oocytes could be possible, if Grade A COC was selected and cultured in the maturation medium supplemented with 10% or 20% FWS.