Cultured aortic endothelial cells, like their in vivo counterparts, form highly organized, confluent monolayers of polarized epithelioid cells that secrete, exclusively at their basal surface, an extracellular matrix to which they then attach. The influence of isolated subendothelial matrix preparations on cell polarity and monolayer organization was studied by presenting fragments of the matrix to confluent bovine aortic endothelial cell cultures. The matrix particles were immediately bound to the apical aspect of the cell monolayer and induced rapid reorganization of the monolayer into cells with a fibroblastoid morphology. To determine if fibronectin, the major glycoprotein of the subendothelial matrix, could be involved in the observed apical cell surface-matrix interactions, latex beads or small discs of Nucleopore filters were coated with the glycoprotein and presented to confluent monolayers. In a fashion similar to that observed with matrix fragments, materials coated with fibronectin caused focal reorganization of the cell layer. After contact with the coated beads, the underlying endothelial cells flowed upward and spread over the entire bead, forming a canopy of confluent cells that draped the particle. Contact of confluent monolayers with the coated filters induced similar behaviour, except that monolayer reorganization into the fibroblastoid phenotype was followed by emigration of the majority of underlying cells through the pores to the upper filter surface, where they formed a new organized cell monolayer with the typical endothelial cell morphology. Thus contact of the apical surface of endothelial cells with structures to which they adhere initiates a rapid disruption of the organized cell monolayer, followed immediately by a concerted effort of the local population to re-establish both cell polarity and monolayer contiguity. The expression of this behaviour may be important during tissue remodeling that occurs in neovascularization and during interactions with thromboemboli.

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