Cumulus expansion, chromatin configuration and meiotic competence in horse oocytes: a new hypothesis

When recovered from the follicle, horse oocytes may be categorised as having either a compact or an expanded cumulus. Cumulus expansion is strongly associated with follicle atresia. Oocytes with expanded and compact cumuli have similar proportions in the germinal vesicle stage when recovered from the follicle. However, during in vitro culture, a higher proportion of oocytes with expanded cumuli mature, and they do so more quickly, than do oocytes with compact cumuli. Using Hoechst 33258 to label chromatin, in the germinal-vesicle stage horse oocytes can be divided into those in which the nucleus fluoresces diffusely (FN), and those in which the chromatin is condensed as a mass within the germinal vesicle (condensed chromatin; CC). The CC configuration is more common in oocytes with expanded rather than compact cumuli. The occurrence of the condensed chromatin configuration in oocytes with compact cumuli rises with increasing follicle size to the point that, in follicles >20 mm diameter, almost 80% of oocytes with compact cumuli have condensed chromatin. This suggests that the chromatin of fully grown oocytes condenses prior to the gonadotrophin stimulation that induces ovulation. The number of oocytes maturing in vitro correlates strongly with the presence of the condensed chromatin configuration at the beginning of culture for both cumulus types. This suggests that the condensed chromatin configuration represents the meiotically competent oocyte. In contrast, FN oocytes appear to be meiotically incompetent, although 61% of oocytes from small, apparently viable follicles, have the FN configuration. This leads to the hypothesis that the FN configuration represents viable germinal vesicle stage oocytes that have yet to acquire meiotic competence. As oocytes achieve meiotic competence, the chromatin appears to condense to the CC configuration. This change occurs during growth of the preovulatory follicle but it can also occur during follicular atresia. Information about the inherent meiotic competence of different populations of oocytes will assist in the selection of oocytes for experiments involving in vitro maturation and fertilisation in equids.