In vitro maturation of oocytes derived from the brown bear (Ursus arctos)

Abstract

This study was conducted to determine whether meiotic maturation could be induced in ovarian oocytes from the American brown bear (Ursus arctos), a model for gamete "rescue" techniques for endangered ursids. The bears were euthanized, and their ovaries were transported to the laboratory within 4 h. The mean ovarian size was 2.4 x 1.8 cm (range: 2.0-3.3 x 1.5-2.2 cm). The ovaries obtained from the 2 brown bears yielded 97 oocytes (48.5/female), and 88 (90.7%) of them were morphologically classified as normal quality. Oocytes were in vitro matured at 38.5 C in 5% CO2 for 24 or 48 h in TCM-199 supplemented with 10% FBS, 1 μg/ml estradiol-17 β, and 10 μg/ml FSH. In Exp. 1, morphologic evaluation of matured oocytes was conducted by measuring the diameters of oocytes with a zona pellucida (ZP) or cytoplasm without a ZP. In Exp. 2, activation was induced by applying two 20 μsec DC pulses of 2.0 kV/cm delivered by an Electro Cell Fusion Generator. The activated oocytes were cultured in TCM-199 containing 2 mM of 6-dimethylaminopurine for 4 h, in Charles Rosenkrans (CR) 1 for 3 days and the in CR2 for another 4 days. The diameters of the matured bear oocytes with a ZP and with cytoplasm without a ZP (161.8 ± 6.0 and 135.3 ± 7.5 μm, respectively) were significantly (P<0.05) larger than those of bovine oocytes (150.7 ± 4.9 and 118.7 ± 7.5 μm). The maturation rates of the bear oocytes were 17.6 and 59.4% at 24 and 48 h of in vitro maturation, the percentage of activated oocytes that developed to the 2 or 4-cell stage was 31.6%; however, no blastocysts were observed. These results indicate that bear oocytes can develop to metaphase II in an in vitro culture system and that activated oocytes can develop to the 2 or 4-cell stages.