PATTERN FORMATION BY OOPLASMIC SEGREGATION IN ASCIDIAN EGGS

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Abstract
This article reviews the generation of cytoplasmic patterns by ooplasmic segregation in ascidian eggs. Ascidian eggs exhibit a spectacular episode of ooplasmic segregation after fertilization which finalizes the position of many different constituents with respect to the future embryonic axes. Among the elements that are rearranged by ooplasmic segregation include three different cytoplasmic regions of unique morphogenetic fate, cytoplasmic organelles, cell surface components, the cytoskeleton, and localized maternal mRNA molecules. New experimental evidence is reviewed suggesting that the reorganization of these constituents into a specific pattern in the egg is coordinated by their association in a cytoplasmic complex. The contraction of an actin filament network in this complex after fertilization is proposed to be the motive force for ooplasmic segregation. The focal point for ooplasmic segregation may be controlled by a local increase in intracellular calcium at or near the point of sperm entry.