CYTOSKELETAL ORGANIZATION OF RAT OOCYTES DURING METAPHASE-II ARREST AND FOLLOWING ABORTIVE ACTIVATION - A STUDY BY CONFOCAL LASER SCANNING MICROSCOPY

In metaphase II arrested rat oocytes (M II), microtubles were found in the taper-shaped meiotic spindle and in the cytoplasm as asters and f ree microtubules. Whereas spindle microtubules were acetylated, those located in the cytoplasm were not. Cytoplasmic microtubules were also labile as assessed by mild cooling. In contast to mouse oocytes, rat m icrotubule organizing centers (MTOCs) did not react with MPM-2 antibod y by immunofluorescence despite the fact that this antibody reacts wit h several proteins as shown by immunoblot. However, cytoplasmic MTOCs in M II-arrested rat oocytes could be detected by their nucleating cap acity in the presence of taxol, a drug that induced the formation of n umerous cytoplasmic asters. In addition, taxol caused a change in the spindle shape and the formation of astral microtubules at the spindle poles. Meiotic spindles (as well as chromosomes devoid of microtubules after nocodazole-treatment) were overlaid by an actin-rich domain. Sp ontaneous abortive activation led to the extrusion of the second polar body followed by another metaphase arrest-metaphase III; however, nor mal spindles did not form and dispersed chromosomes surrounded by micr otubles were observed. Electron microscopic studies confirmed these ob servations and revealed that the kinetochores are located deep within the chromosomes in contrast to mouse kinetochores, and this might be r esponsible for the absence of a metaphase III spindle in the rat oocyt e. Induced activation caused transition to interphase with the formati on of a characteristic microtubule network. This study shows that ther e are several significant differences in the cytoskeletal organization of rat and mouse oocytes.