We have studied the behavior of cloned capillary endothelial cells grown inside a threedimensional collagen matrix. Cell monolayers established on the surface of collagen gels were covered with a second layer of collagen. This induced the monolayers of endothelial cells to reorganize into a network of branching and anastomosing capillary-like tubes. As seen by electron microscopy, the tubes were formed by at least two cells (in transverse sections) delimiting a narrow lumen. In addition, distinct basal lamina material was present between the abluminal face of the endothelial cells and the collagen matrix. These results showed that capillary endothelial cells have the capacity to form vessel-like structures with well-oriented cell polarity in vitro. They also suggest that an appropriate topological relationship of endothelial cells with collagen matrices, similar to that occurring in vivo, has an inducive role on the expression of this potential. This culture system provides a simple in vitro model for studying the factors involved in the formation of new blood vessels (angiogenesis).

The endothelial lining of blood capillaries consists of a single layer of flattened cells whose basal surface interacts with the extracellular matrix, while the apical surface delimits the lumen of the vessel. The important role played by microvascular endothelium in a wide range of normal and pathological processes has stimulated the development of culture methods for the growth of capillary endothelial cells in vitro (1-3, and references therein). These studies have been so far carried out on two-dimensional substrata, on which capillary endothelial cells usually grow as typical monolayers, although a tendency to form tubular structures has been reported in some conditions (4).