Fine structure of equine oocytes matured in vitro for 15 hours

Transmission electron microscopy (TEM) was used to evaluate the fine structure of equine oocytes cultured in vitro. Oocytes obtained by follicular aspiration were cultured for either zero or 15 hr. After treatment oocytes were processed either by light microsocopy (nuclear evaluation) or TEM (cytoplasmic evaluation). Those oocytes cultured for 15 hr were incubated in modified TCM 199 with 15% (v/v) mare serum (day of ovulation) at 39 ± 0.2°C. Evaluation using TEM revealed that cortical granules were present in all oocytes. However, zero‐time oocytes contained few cortical granules, and these were scattered throughout the cytoplasm, whereas 15 hr oocytes contained numerous cortical granules primarily found in very close proximity to the oolemma. Further ultrastructural analysis of both groups revealed organelle structure similar to that previously described for in vivo matured equine oocytes. Evaluation of nucelar maturity (lacmoid stain) showed that 15 hr of culture resulted in significant numbers of oocytes at metaphase II (8/17; 47%). These data demonstrate that oocytes cultured for 15 hr in modified TCM 199 with 15% mare serum (day of ovulation) are mature with respect to nuclear configuration and cortical granule migration and, therefore, would be appropriate candidates for in vitro fertilization. © 1994 Wiley‐Liss, Inc.