Collagen-based matrices with stem cells

A tissue graft composition, said composition comprising:

• an engineered, purified collagen-based matrix comprising collagen fibrils, wherein the matrix has a collagen fibril microstructure including fibril-fibril branching;
• and
• a network of lumenized, endothelial cell-lined vessels formed in vitro, said vessels capable of persisting for more than 7 days in vitro;
• wherein said collagen fibrils are polymerized from a pig skin collagen source comprising a mixture of type I collagen monomers and oligomers, wherein the concentration of collagen in the polymerization reaction is 0.3 mg/ml to 1 mg/ml, wherein the vessels are formed from endothelial progenitor cells, and wherein the matrix has a shear storage modulus of 40 Pa to 50 Pa and a fibril volume fraction of 5% to 10%;
• and
• wherein the tissue graft composition is produced by engineering the purified collagen-based matrix, and seeding the matrix at a density of 1×105 to 5×105 endothelial progenitor cells per milliliter to form one or more vessels within the matrix.