Protrusion-Retraction Dynamics of an Annular Lamellipodial Seam

Abstract

The geometry of single cells (as leukocytes or keratinocytes) spreading on a surface like a “fried egg” (Winklbauer et al. I.1this volume) is characterized by a dense cortical layer which surrounds the whole cell body and, near the substratum, forms a more or less pronounced ring, visible e.g. in Fig. l A of Hinz & Brosteanu (I.2 this volume). Mainly consisting of actin filaments crosslinked via myosin or other pro­teins, and more or less closely connected to the plasma membrane, this contractile cortex (CC) is an obvious candidate for performing two important biomechanical functions of the cell’s “statics and dynamics”, namely the following counteractions: (1) to provide the force for extending various membrane protrusions (filopods, ruf­fles, lamellipods or even blebs — having different functions within the processes of adhesion and locomotion) and, (2) to hold or withdraw these protrusions, thereby determining the shape of the cell and guaranteeing its integrity. Indeed, while not undergoing cell division (He & Dembo I.7, Svetina et al. I.8 this volume), a typical 3-dimensional cell in its contracted state is nearly ball-shaped, so that for this ho­mogeneous situation we would, according to usual mechanical concepts, presume (1) the existence of a (nearly constant) intracellular hydrostatic pressure (P _B ) which, by bulging out folds and ruffles within the membrane, provides an effective outward normal force onto the “cell boundary”, but also (2) a (nearly constant) “effective surface tension” which prevents the occurrence of “fingering” instabili­ties with increasing negative (concave) curvature of the cell periphery. However, for a closer understanding of the underlying molecular mechanisms, details of the dynamics within each of the different protrusions (such as filopods, lamellipods, or blebs) have to be investigated and modelled; for a review and further references see Discussion & Open Problems of Chapter I.