EPITHELIAL FOLDING IN-VITRO - STUDIES ON THE CELLULAR MECHANISM UNDERLYING EVAGINATION OF THYROCYTE MONOLAYERS

Epithelial monolayers in suspension culture fold in a way which closel y resembles epithelial evagination. We have used freshly isolated segm ents of porcine thyroid follicles to study the mechanism underlying th is evagination process. Epithelial folding was accompanied by dramatic changes in cell shape: the cells elongated and apical cell surfaces w idened, whereas the basal cell portions were narrowed to about 20% of their original width. Apparently, enzymatic separation of thyroid epit helial 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+ depende nt and reversibly blocked by cytochalasin D, by which the reorganizati on of the F-actin network was disturbed. This inhibitory effect was al so observed by the action of cAMP analogues known to cause rounding of cells by their effect on cortical F-actin. Moreover, evagination in v itro was reversibly blocked at intracellular pH values of 5.8 and belo w. Under these conditions, protein phosphorylation was entirely inhibi ted. 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 es sential for folding. We conclude that enzymatic separation of epitheli al monolayers from their extracellular matrix initiated a cascade cons isting of extended cell-cell interactions of the lateral plasma membra nes and of reorganization of the apical actin-myosin network, finally resulting in profound changes in cell shape characteristic of epitheli al evagination. (C) 1997 Academic Press.