In vitro maturation of horse oocytes: characterization of chromatin configuration using fluorescence microscopy

The chromatin configuration of resting horse oocytes and the time course of in vitro oocyte maturation was characterized using a fluorescent, DNA-specific label. Oocytes were classified as having either compact (CP) or expanded (EX) cumuli at the time of collection. Centrifugation of oocytes was effective in allowing visualization of the germinal vesicle. Two main chromatin configurations were found in oocytes known to have a germinal vesicle: condensed chromatin (CC), in which the chromatin formed a dense mass surrounding the nucleolus; and fluorescing nucleus (FN), in which the entire nucleus, containing diffuse or spotty chromatin, was visible. The proportion of CC to FN was higher for oocytes with EX cumuli. At time 0, 78% of CP oocytes and 73% of EX oocytes were in the germinal vesicle stage. Significantly more EX than CP oocytes were in metaphase I or II at time 0. In both CP and EX groups, maturation had not begun after 8 h of incubation. Maximal maturation occurred after 24 h for oocytes in the EX group, whereas CP oocytes continued to mature between 24 and 32 h. The percentage of EX oocytes in metaphase I did not change between 24 and 32 h, indicating a possible arrest of some EX oocytes at metaphase I. There was no difference in the percentage of oocytes at metaphase II between the CP and EX groups after 32 h of incubation.