Sea urchin oocytes possess elaborate cortical arrays of microfilaments, microtubules, and intermediate filaments

Extensive arrays of microfilaments, microtubules and cytokeratin-type intermediate filaments were detected in the cortex of Strongylocentrotus droebachiensis oocytes using fluorescently labeled antibodies on both cortex and whole mount preparations. All three filament systems undergo dramatic structural reorganization during meiotic maturation of the egg. Microfilaments form a dense meshwork within the cortex of the oocyte. After meiosis, the filaments rearrange and shorten, resulting in a more loosely organized network. Both cortical microtubules and microtubules associated with a microtubule-organizing center are observed within the oocyte. After meiosis, the number and length of the cortical microtubules gradually diminish. A microtubule organizing center is found situated between the germinal vesicle and the plasma membrane in many oocytes. A network of filaments extends from the microtubule organizing center and radiates peripherally toward the germinal vesicle, presumably marking the animal pole. Cytokeratin-like intermediate filaments form a reticular network within the oocyte cortex, then solubilize during meiosis. In whole mounts of oocytes there is a single focal center of cytokeratin staining from which filaments radiate. Indirect immunofluorescence experiments, using anti-tubulin and anti-cytokeratin antibodies simultaneously, reveal the intermediate filament focal center to be localized within the microtubule organizing center. These results demonstrate the presence of a complex cortical cytoskeleton in premeiotic eggs of the sea urchin, Strongylocentrotus droebachiensis.