We have used quick-freezing, freeze-fracture, and deep etching to study postfertilization growth of microvilli in the sea urchin, Strongylocentrotus purpuratus. Between 1 and 2 min postinsemination, we observed rapid growth of numerous finger-like microvilli to about 0.7 μm in length. Subsequently, groups of two to four microvilli became raised off the egg surface on broad cytoplasmic bases. Finally, by 5 min postinsemination, these bases had become interconnected by thin sheets of cytoplasm to form a continuous network of ruffles across the egg surface. Corresponding thin sections of quick-frozen and freeze-substituted eggs showed that these microvilli contained networks of microfilaments. The finger-like processes had bundles of parallel microfilaments that ran their entire length, while the ruffles of cytoplasm contained sheets of crisscrossed microfilaments. These observations support the hypothesis that microvillar growth results from formation of new microfilament networks. © 1981.
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