Organization of actin filaments in early chick embryo ectoderm: an ultrastructural and immunocytochemical study.

By combining transmission electron microscopy (TEM), scanning electron microscopy (SEM) and immunocytochemistry at both light and TEM levels the organization of actin in early chick ectoderm has been demonstrated. Every cell is encircled by a ring of actin filaments which are associated with apically situated intercellular junctions. In this way the actin ring of each individual cell is connected to that of surrounding cells and a continuous network is formed. As clearly shown by immunofluorescence and SEM studies on intact and Triton X-100 treated whole-mount preparations this network extends throughout the entire layer, interrupted only at the blastoderm margins and the primitive streak. A more diffuse arrangement was sometimes seen at cell bases. It is suggested that the network extending throughout the ectoderm gives it stability and cohesion and is important in gross morphogenetic movements involving the entire layer. The less-organized arrangement at cell bases is associated more with movements of individual cells.