COMPETENT MOUSE OOCYTES ISOLATED FROM ANTRAL FOLLICLES EXHIBIT DIFFERENT CHROMATIN ORGANIZATION AND FOLLOW DIFFERENT MATURATION DYNAMICS

After labelling DNA with the specific vital fluorophore Hoechst 33342, oocytes, isolated by puncture from antral follicles in adult mice, ha ve two essentially different configurations of their nuclear fluoresce nce images. These have been called SN (where the nucleolus is surround ed by chromatin) and NSN (where the nucleolus is not surrounded by chr omatin). Intermediate configurations are also found, although with a l ower frequency. The proportion of each class is on the average equal a nd depends neither on the presence of cumulus cells nor on the age of the mouse. Electron microscopy confirms several ultrastructural differ ences between these two nuclear configurations, namely, the structure of the nucleolus, which is vacuolated in NSN-type and compact in SN-ty pe oocytes. Using video-enhanced fluorescence microscopy at low level of excitation light, we could follow directly in vitro the meiotic mat uration of both classes, without impairing their viability. We show th at in germinal vessicle (GV) state, the chromatin does not change from one configuration into the other and that both classes are able to ma ture to metaphase II, although the maturation has slightly different c haracteristics. (C) 1993 Wiley-Liss, Inc.