Role of actin filaments in shape formation of mesenteric mesothelial cells of the bullfrog.

The role of actin filaments in the development of cellular shape in the mesenteric mesothelium of the bullfrog was studied by using a simple, new technique for making en face preparations of mesothelial sheets. By using these mesothelial cell preparations, the distribution of actin was determined by means of fluorescence microscopy with 7-nitrobenz-2-oxa-1,3-diazole (NBD)-phallacidin and that of myosin by means of immunofluorescence microscopy. Although fluorescence produced by both NBD-phallacidin and antimyosin staining was found exclusively along the margins of the cells, its intensity was altered in correspondence with changes in cell shape. For instance, tadpole-type mesothelial cells with either an irregular or very slender cell shape showed very weak fluorescence. On the other hand, frog-type mesothelial cells with a polygonal shape showed intense fluorescence at their margins and had circumferential bundles of actin filaments at their apices. Furthermore, intercellular junctions between the mesothelial cells developed as the cell shape became polygonal during metamorphosis. The present study showed that development of circumferential bundles of actin filaments and intercellular junctions may serve to establish and maintain the definitive polygonal cellular pattern in the mesenteric mesothelium of the bullfrog.