In vivo and in vitro maturation of oocytes collected from superstimulated wood bison (Bison bison athabascae) during the anovulatory and ovulatory seasons

- Wood bison oocytes were induced to mature both in vivo and in vitro.
- In vivo oocyte maturation was more complete at 30 h vs. 24 h after hCG treatment.
- COC expansion was more complete and more were fully expanded after in vivo maturation.
- Nuclear maturation occurred more rapidly during in vitro vs. in vivo maturation.
- Season had no effect on the maturational capacity of wood bison oocytes.

Experiments were done to compare the in vivo and in vitro maturational characteristics of cumulus-oocyte complexes (COC) collected from live wood bison. In Experiment 1 (anovulatory season), follicular ablation was done to synchronize follicle wave emergence among bison on Day −1, and FSH was given on Days 0 and 2. Bison were then assigned to 5 groups (n = 5/group) in which COC were collected by transvaginal follicle aspiration on Day 4 and either fixed immediately with no maturation (control), matured in vitro for 24 or 30 h, or collected on Day 5 after in vivo maturation for 24 or 30 h (i.e., after hCG treatment). In Experiment 2 (ovulatory season), bison were treated as described for Experiment 1, but PGF2α (cloprostenol) was given to control the luteal phase on Days −9 and 3. In both experiments, cumulus cell expansion was more extensive following in vivo than in vitro maturation, and the percentage of fully expanded COC was highest in the in vivo 30 h groups. Nuclear maturation occurred more rapidly in vitro; 60-70% of oocytes were at the MII stage 24 h after in vitro maturation while only 25-27% of oocytes had reached the MII stage after 24 h of in vivo maturation. In conclusion, nuclear maturation occurred more rapidly during in vitro vs. in vivo maturation, but was associated with less cumulus expansion than in vivo maturation. In vivo oocyte maturation was more complete at 30 vs. 24 h after hCG treatment. Season had no effect on the maturational capacity of wood bison oocytes.