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The reorganization of cytoskeletal fibre systems in spreading porcine endothelial cells in cultures.

Porcine aortic endothelial cells spreading on a glass substrate undergo characteristic changes in shape which can be classified into four distinct stages. To study the role of the cytoskeleton in cell spreading, we have examined the distribution of microtubules (MT), microfilaments (MF), and intermediate filaments (IF) at each of these stages by using immunofluorescence and antisera specific for tubulin, tropomyosin, myosin, and vimentin. The small round Stage I cells showed diffuse staining with four antisera. In the more flattened spreading Stage II cells, MT and IF were first observed in the perinuclear region while fibres straining positively for tropomyosin and myosin were first seen along the cell margin. Later the MT began to radiate out in all directions from the perinuclear region while the IF became localized in a region on one side of the nucleus. In the very flat Stage III cells with a circular outline, additional MT could be seen along and parallel to cell margin while the IF emanating from the perinuclear region and the tropomyosin and myosin positive fibres became concentrically distributed around the nucleus. In the very flat asymmetric Stage IV cells, both the MT and IF radiated out from the perinuclear region towards the cell periphery while most of the tropomyosin and myosin-positive fibres became reorganized so that they ran parallel to the edges of the cell. In addition several loci from which a number of the tropomyosin and myosin-containing fibres radiated also appeared at this stage. These results indicate that during spreading each of the three major fibre systems undergo extensive and specific reorganization which is well coordinated with changes in cell shape.