The changes in the alignment of F-actin bundles in epithelial cells were studied, mainly using NBD-phallacidin, as the isolated corneal epithelium started to spread along the surface of Millipore filter. The epithelial cells at the margin were found to make the transition from a stationary to a motile state in two steps: first, from polygonal cells, in which F-actin was deposited along the cell border, to hemispherical cells, in which F-actin was distributed homogeneously along the inner cell surface; and second, from the hemispherical cells to spread cells in which F-actin bundles ran radially or in parallel to each other through the leading lamella from a cytocentre to the edges, as in fibroblastic cells. During the second step, an amorphous mass of F-actin was transiently accumulated at the distal end in the half-extended lamella and structured into bundles during development of the lamella. Since this proceeded only on substrata to which marginal cells could make contact firmly enough to enable the cells to extend, it was concluded that F-actin bundles were aligned within extending lamellae in relation to the tension exerted by the lamellae, and might be associated with other components to be microfilaments.

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