Spreading of vascular endothelial cells in culture: spatial reorganization of cytoplasmic fibers and organelles.

The three-dimensional organization and fine structure of cytoplasmic components within whole non-embedded bovine aortic endothelial cells were examined during their attachment and spreading in tissue culture. Cells were cultured directly on Formvar-coated gold grids, fixed in glutaraldehyde and osmium tetroxide, critical point dried and examined by transmission electron microscopy (TEM) using stereoscopic methods, and by scanning electron microscopy (SEM). Reorganization of cytoplasmic structures during cell spreading occurred in four sequential stages: (1) spreading of the plasma membrane and unstructured cytoplasmic matrix; (2) spreading of cytoplasmic fiber systems (microtubules, microfilament bundles and microtrabecular system); (3) alignment of microfilament bundles and formation of radial tracts of microtubules; and (4) centripetal movement of organelles along radial tracts. These stages observed by TEM correlated with progressive degrees of cell flattening as visualized by SEM. These studies demonstrate that a characteristic reorganization of intracellular fiber systems and organelles accompanies the spreading of endothelial cells in culture.