Oocyte meiotic maturation involves complex processes of nuclear breakdown, spindle formation, and organelle movement necessary for successful fertilization and early development. Yet these take place in the mouse without the aid of centrioles; instead the spindle is organized by a number of microtubule organizing centers (MTOC).Multiple MTOC are seen by fluorescence microscopy to migrate from the cortex to the nucleus (germinal vesicle; GV) during the first 30 min of maturation. However, EM has not revealed the expected ultrastructure of MTOC known from previous studies. New information from the present study sheds light on MTOC number, location and ultrastructure prior to and during very early maturation.

Maturation occurs in vitro when oocytes are released from ovarian follicles into culture, but is blocked by dibutyrl cAMP or IBMX (3-isobutyl-l-methyl xanthine; Sigma). MTOC were studied after 0-2 h in culture medium; IBMX medium (0.2 mM); an inhibitor of MT (nocodazole, lμM); or MF (cytochalasin, 5 μg/ml) by fluorescent confocal microscopy (CM; Biorad 600) or standard EM.