Spreading and locomotion of tissue cells: factors controlling the distribution of pseudopodia.

The spreading and locomotion of cells on substrata can be regarded as a result of the interaction of two groups of processes: pseudopodial attachment and stabilization. Stabilization processes integrate the results of previous pseudopodial reactions and determine the sites of further extensions. Stabilization mechanisms are probably based on the changes in the distribution of cytoskeletal elements. Usually the direction of pseudopodial extensions is approximately parallel to the predominant orientation of actin microfilaments in the nearby cortex. Two variants of stabilization can be distinguished: microtubule-independent and microtubule-dependent processes. Contact paralysis of the upper surfaces of epithelial sheets is possibly a special case of microtubule-independent stabilization. In the course of spreading, the cell may acquire a polarized or discoid shape depending on the efficiency of attachment.