The organization of actin filaments in the sand dollar egg cortex was studied by fluorescence microscopy using an F-actin specific stain, NBD-phallacidin (NBD-Ph). Cortices were prepared from Clypeaster japonicus eggs which had been stuck down onto polycation-coated glass surfaces and sheared with a stream of a medium capable of preserving actin-containing structures. Cortices from unfertilized eggs showed weak fluorescence with NBD-Ph. Immediately after fertilization, however, isolated cortices were brightly fluorescent. Fluorescent spikes which seemed to correspond to actin filament bundles in microvilli were prominent structures of cortices isolated before and at the streak stage. At anaphase, a fluorescent meshwork which seemed to consist of actin filament bundles was observed all over the isolated cortex. At telophase, the contractile ring was observed as brightly fluorescent oriented fibers in the equatorial cortex. The fluorescent meshwork still remained in the rest of the cortex. Soon after cleavage, the fluorescent meshwork became obscure and fluorescent spikes appeared again. When whole eggs were stained with NBD-ph, the fibrous structures were not identifiable, although the changes in the fluorescence intensity of the cortical layer were consistent with those in isolated cortices. The process of formation of the contractile ring is discussed. © 1986.
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