Light microscope observations on actin distribution during morphogenetic movements in the chick embryo*

The cellular distribution of actin during two morphogenetic processes in the chick embryo has been observed, using a high-resolution fluorescent technique, with heavy meromyosin as a probe. These cytoskeletal elements have been implicated in all cell and tissue movements in the embryo. It is now commonly accepted that microfilaments are necessary to provide the motive force for morphogenesis. Two morphogenetic movements in the early embryo have been studied at the light microscope level. During somitogenesis, the mesenchymal segmental plate becomes transformed into a meristic series of transient epithelial vesicles. Again, actin distribution is diffuse and random before the morphogenetic event. During epithelialization, actin becomes prominent in the apical regions of the epithelial cells. Cells in the somitic epithelial vesicles, the core cells, appear to be passive participants in this process, and consequently show no distinct cellular localization of actin.