Fine structure of the follicular oocyte of the horse

Oocytes recovered by follicular aspiration were evaluated by light and transmission electron microscopy. Of the 22 oocytes, 4 exhibited characteristics of degeneration, and the remaining 18 were in various stages of meiotic development. Of the non-degenerate oocytes, 14 were in the germinal vesicle stage, 2 had undergone nuclear membrane disintegration, 1 displayed chromosomes in late metaphase I-early anaphase I, and 1 oocyte was in the process of extrusion of the first polar body. Although some oocytes retained complete cumulus cell investments, oocytes were predominantly enclosed only by the corona radiata. Ultrastructural evaluation revealed that follicular oocytes of the horse are similar to those of other mammals. An abundance of vesicular agranular endoplasmic reticulum in horse oocytes was the most obvious difference. Variation in intracellular organization seemed to be primarily dependent upon stage of meiotic development.