Appearance in confluent vascular endothelial cell monolayers of a specific cell surface protein (CSP-60) not detected in actively growing endothelial cells or in cell types growing in multiple layers.

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Abstract
The formation of a highly organized [bovine] vascular and corneal endothelial cell monolayer is associated with the appearance of 60,000-dalton cell surface protein (CSP-60) (30,000 daltons after reduction with dithiothreitol) which is not detectable in rapidly growing endothelial cells and in subconfluent cultures that do not yet exhibit the strict morphology of a confluent monolayer. It is also absent from vascular smooth muscle cells and from endothelial cultures that are maintained in the absence of fibroblast growth factor and grow on top of each other at confluence. After disorganization of cells in a confluent endothelial monolayer by urea, EDTA, or trypsin, CSP-60 is no longer exposed on the cell surface, but it reappears as soon as the cells readopt their characteristic 2-dimensional configuration. This reorganization can be achieved in the presence of cycloheximide and despite removal of fibronectin by urea, EDTA or trypsin. Maximal amounts of fibronectin and no CSP-60 are detected in subconfluent, but not yet organized, endothelial cultures or in endothelial cells that no longer form a monolayer of nonoverlapping cells at confluence. Cultures of vascular smooth muscle cells contain fibronectin but no CSP-60. CSP-60, rather than fibronectin, could be involved in the adoption of a monolayer confuguration by confluent endothelial cells.