Epithelial folding in vitro: studies on the cellular mechanism underlying evagination of thyrocyte monolayers.

Epithelial monolayers in suspension culture fold in a way which closely resembles epithelial evagination. We have used freshly isolated segments of porcine thyroid follicles to study the mechanism underlying this evagination process. Epithelial folding was accompanied by dramatic changes in cell shape: the cells elongated and apical cell surfaces widened, whereas the basal cell portions were narrowed to about 20% of their original width. Apparently, enzymatic separation of thyroid epithelial cells from their underlying extracellular matrix resulted in an extension of the lateral cell-cell interactions on the expense of the basal cell surface area. Epithelial folding in vitro was Ca2+ dependent and reversibly blocked by cytochalasin D, by which the reorganization of the F-actin network was disturbed. This inhibitory effect was also observed by the action of cAMP analogues known to cause rounding of cells by their effect on cortical F-actin. Moreover, evagination in vitro was reversibly blocked at intracellular pH values of 5.8 and below. Under these conditions, protein phosphorylation was entirely inhibited. Inhibitors of protein kinases, specifically of myosin light chain kinase, were able to disrupt the evagination process, suggesting that protein phosphorylation, presumably of the myosin light chain, was essential for folding. We conclude that enzymatic separation of epithelial monolayers from their extracellular matrix initiated a cascade consisting of extended cell-cell interactions of the lateral plasma membranes and of reorganization of the apical actin-myosin network, finally resulting in profound changes in cell shape characteristic of epithelial evagination.