In vitro maturation of horse oocytes.

Ovaries collected from slaughtered mares of unknown reproductive history were transported to the laboratory, and their oocytes were recovered and cultured in modified TCM 199 supplemented with 20% horse serum and additional granulosa cells. To characterize the ovaries, the size and number of follicles were counted. To determine the time required for nuclear maturation, oocytes were fixed either after 18 h (n=23), 24 h (n=50), or 30 h (n=33) of culture. After co-culture with granulosa cells most oocytes reached metaphase II (M II) by 30 h (72.7%). After 24 h of maturation only 56.0% of the cultured oocytes had reached metaphase II (M II).