Increasing culture time from 48 to 96 or 144 hours increases the proportions of equine cumulus oocyte complexes with negative or fragmented nucleus morphology

Abstract

The objective was to test the hypothesis that increasing equine oocyte culture time from 48 to 96 or 144 h increases nucleus maturation of equine oocytes. The hypothesis was not supported because condensed chromatin-stage oocytes decreased (P < 0.01) from 33 126 (26.2%) at 48 h or 34 95 (35.8%) at 96 h to 11 117 (9.4%) at 144 h, and polar body-stage oocytes decreased (P < 0.01) from 65 126 (51.6%) at 48 h to 25 95 (26.3%) at 96 h and (P < 0.01) to 1 117 (0.9%) at 144 h. Negative (non-staining) oocytes increased (P < 0.01) from 16 126 (12.7%) at 48 h or 15 95 (15.8%) at 96 h to 39 117 (33.3%) at 144 h. Fragmented oocytes (with and without fluorescent areas) increased (P < 0.01) from 4 126 (3.2%) at 48 h to 20 96 (21.1%) at 96 h and increased again to 60 117 (51.3%) at 144 h. When fragmented oocytes having 1 fluorescent area were defined as condensed chromatin-stage and fragmented oocytes having 2 fluorescent areas were defined as polar body-stage, condensed chromatin-stage oocytes increased (P < 0.05) from 34 126 (27.0%) at 48 h to 38 95 (40.0%) at 96 h, but decreased (P < 0.05) to 19 117 (16.2%) at 144 h. Polar body-stage oocytes decreased (P < 0.01) from 66 126 (52.4%) at 48 h to 27 95 (28.4%) at 96 h and decreased again to 7 117 (6.0%) at 144 h. Fragmented oocytes without any fluorescent areas increased (P < 0.01) from 2 126 (1.6%) at 48 h to 14 95 (14.7%) at 96 h and increased again to 46 117 (39.3%) at 144 h. Under the conditions of this experiment, the hypothesis that increasing the culture time of equine oocytes from 48 to 96 or 144 h would increase oocyte maturation was not supported. We propose that the culture system needs to be improved before this hypothesis can be adequately tested, because prolonged culture significantly increased the portions of negative and fragmented equine oocytes.