Actin, microvilli, and the fertilization cone of sea urchin eggs.

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Abstract
Sea urchin [Arbacia punctulata] eggs and oocytes at the germinal vesicle stage were fixed at various times after insemination, and thin sections were examined. Actin filaments were first visible in the cortical cytoplasm 1 min after insemination, and by 2 min enormous numbers of filaments are present. At these early stages, the filaments are only occasionally organized into bundles, but one end of many filaments contacts the plasma membrane. By 3 min, and even more dramatically by 5 min after insemination, the filaments become progressively more often found in bundles that lie parallel to the long axis of the microvilli and the fertilization cones. By 7 min, the bundles of filaments in the cone are maximally pronounced, with virtually all the filaments lying parallel to one another. Decoration of the filaments with subfragment 1 of myosin shows that, in both the microvilli and the cones, the filaments are unidirectionally polarized with the arrowheads pointing towards the cell center. The efflux of H+ from the eggs was measured as a function of time after insemination. The rapid phase of H+ efflux occurs at the same time as actin polymerization. The formation of bundles of actin filaments in microvilli and in cones is apparently a 2-step process, involving actin polymerization to form filaments, randomly oriented but in most cases having 1 end in contact with the plasma membrane followed by the zippering together of the filaments by macromolecular bridges.