Organization and regulation of cortical microtubuies during the first cell cycle of Xenopus eggs

Anti-tubulin antibodies and confocal immunofluorescence microscopy were used to examine the organization and regulation of cytoplasmic and cortical microtubuies during the first cell cycle of fertilized Xenopus eggs. Appearance of microtubuies in the egg cortex temporally coincided with the outgrowth of the sperm aster. Microtubuies of the sperm aster first reached the animal cortex at 0.25, (times normalized to first cleavage), forming a radially organized array of cortical microtubuies. A disordered network of microtubuies was apparent in the vegetal cortex as early as 0.35. Cortical microtubule networks of both animal and vegetal hemispheres were reorganized at times corresponding to the cortical rotation responsible for specification of the dorsal-ventral (D-V) axis. Optical sections suggest that the cortical microtubuies are continuous with the microtubuies of the sperm aster in fertilized s* o r a n extensive activation aster in activated eggs. Neither assembly and organization, nor disassembly of the cortical microtubuies coincided with MPF activation during mitosis. However, cycloheximide or 6-dimethylaminopurine, which arrest fertilized eggs at interphase, blocked cortical microtubule disassembly. Injection of pl3, a protein that specifically inhibits MPF activation, delayed or inhibited cortical microtubule breakdown. In contrast, eggs injected with cycA90, a truncated cyclin that arrests eggs in M-phase, showed normal microtubule disassembly. Finally, injection of partially purified MPF into cycloheximide-arrested eggs induced cortical microtubule breakdown. These results suggest that, despite a lack of temporal coincidence, breakdown of the cortical microtubuies is dependent on the activation of MPF.