ORIENTATION OF SPINDLE AXIS AND DISTRIBUTION OF PLASMA-MEMBRANE PROTEINS DURING CELL-DIVISION IN POLARIZED MDCKII CELLS

MDCKII cells differentiate into a simple columnar epithelium when grow n on a permeable support; the monolayer is polarized for transport and secretion. Individual cells within the monolayer continue to divide a t a low rate without disturbing the function of the epithelium as a ba rrier to solutes. This presents an interesting model for the study of mitosis in a differentiated epithelium which we have investigated by c onfocal immunofluorescence microscopy. We monitored the distribution o f microtubules, centrioles, nucleus, tight junctions, and plasma membr ane proteins that are specifically targeted to the apical and basolate ral domains. The stable interphase microtubule cytoskeleton was rapidl y disassembled at prophase onset and reassembled at cytokinesis. As th e interphase microtubules disassembled at prophase, the centrioles mov ed from their interphase position at the apical membrane to the nucleu s and acquired the ability to organize microtubule asters. Orientation of the spindle parallel to the plane of the monolayer occurred betwee n late prophase and metaphase and persisted through cytokinesis. The c leavage furrow formed asymmetrically perpendicular to the plane of the monolayer initiating at the basolateral side and proceeding to the ap ical domain. The interphase microtubule network reformed after the cen trioles migrated from the spindle poles to resume their interphase api cal position. Tight junctions (ZO-1), which separate the apical from t he basolateral domains, remained assembled throughout all phases of mi tosis. E-cadherin and a 58-kD antigen maintained their basolateral pla sma membrane distributions, and a 114-kD antigen remained polarized to the apical domain. These proteins were useful for monitoring the chan ges in shape of the mitotic cells relative to neighboring cells, espec ially during telophase when the cell shape changes dramatically. We di scuss the changes in centriole position during the cell cycle, mechani sms of spindle orientation, and how the maintenance of polarized plasm a membrane domains through mitosis may facilitate the rapid reformatio n of the polarized interphase cytoplasm.