Abstract: The present invention relates to the development of new blood vessels. More specifically, this invention relates to compositions and methods for forming cultured endothelial cells into tubes within a three-dimensional gel. This invention also relates to implanting the resultant gels into animals wherein the tubes undergo remodeling into complex microvessels lined by the endothelial cells. The compositions and methods of the present invention have applications in all aspects of tissue and organ transplantation and grafting. The invention finds particular use in the grafting of engineered skin onto recipients with impaired vascularization. In addition, the present invention identifies genes and gene products which are differentially expressed in immature, maturing and mature microvessels.