Polarity of actin at the leading edge of cultured cells

Abstract

VERTEBRATE non-muscle cells are known to contain considerable amounts of actin and myosin^1–3, but the mechanisms underlying their motility have yet to be elucidated. Various theories have been proposed^1,4–7 to explain the well known phenomena of cell migration, membrane ruffling and unidirectional growth processes, different emphasis being placed on the involvement of membrane components and contractile filament assemblies. These hypotheses have been limited, however, by the lack of information about the detailed structural organisation of the motile regions of the cytoplasm. As one approach to this problem we have developed procedures for the direct observation of the filamentous components of the lamella regions of cultured cells in the electron microscope^8,9. The method, similar to that adopted by Brown et al.¹⁰, consists of the negative-staining of cells grown directly on electron microscope grids after removal of the cell membranes with Triton X-100. By this means, we demonstrate here a single polarity of actin at the leading edge of cultured cells, and consider its implications with regard to cell motility.