Isolated vegetal cortex from Xenopus oocytes selectively retains localized mRNAs.

The process of converting an egg to a multicellular organism often begins with a polarization of information within the oocyte. The cytoskeleton plays an important role in maintaining such asymmetries. Here we describe the isolation and characterization of cortices, rich in cytoskeletal elements, from the animal and vegetal poles of stage VI Xenopus oocytes. Ultrastructural and immunocytochemical analyses revealed that the 10-microns-thick isolates included a cytokeratin network, mitochondria, and other elements, found in the oocyte cortex. Furthermore, three vegetally localized mRNAs, Vg1, Xcat-2, and Xcat-3, were all found to be specifically retained within the isolated vegetal cortices, in sharp contrast to histone RNA. Our findings support the view that the vegetal cortex represents a unique cytoskeletal domain in which a rare class of maternal mRNAs becomes localized.