Initial take, development, and function of transplanted engineered tissue substitutes are crucially dependent on
rapid and adequate blood perfusion. Therefore, the development of rapidly and efficiently vascularized tissue
grafts is vital for tissue engineering and regenerative medicine. Here we report on the construction of a network
of highly organotypic capillaries in engineered tissue substitutes. We employed a three-dimensional culture
system consisting of human microvascular endothelial cells. These were reproducibly expanded at high purity
and subsequently seeded into biodegradable, fibrin-based hydrogels. The process of capillary formation in vitro
followed the principles of both angiogenesis and postnatal vasculogenesis and a distinct sequence of other
developmental steps that closely resemble embryonic neovascularization. Capillary lumen formation in vitro was
initiated by the deposition of a basement membrane and intensive pinocytosis, followed by the generation of
intracellular vacuoles, successive fusion of these vacuoles, and finally the formation of a long, continuous lumen.
After transplantation the vascular structures were stabilized by mural cells of the recipient animal. Our findings
suggest that the in vitro engineering of prevascularized matrices is within reach.