Arrangement of actin filaments and cytoplasmic granules in the sea urchin egg after TPA treatment.

Elongation of microvilli and formation of actin filaments after treatment with a phorbol ester, TPA, were investigated in unfertilized eggs of Hemicentrotus pulcherrimus. Microvilli on the egg surface were examined by scanning electron microscopy. Actin filaments in the cortical layer of the eggs were observed by fluorescence microscopy using rhodamine-labeled phalloidin. The actin molecules were polymerized and bundled to form long filaments inside the cortical layer of eggs after TPA treatment. Arrangement of the actin filaments was followed by spiral elongation of microvilli. Transmission electron microscopic studies showed that the cortical granules under the cell membrane of sea urchin eggs were transferred after TPA treatment from the surface to the interior of the cell [Ciapa et al., 1988: Dev. Biol. 128:142-149]. This movement of the cortical granules was inhibited by cytochalasin B, but not by nocodazole. Furthermore, the distribution of clear granules was changed following TPA treatment. From these results we conclude that intracellular actin filaments may cause the transport of cortical granules and clear granules into the central area of the egg by the activation of protein kinase C. The possible involvement of actin in the inward displacement of granules might be the result of the rearrangement of actin filaments in the cortical layer.