Scaffolds can be populated with adult-derived cells that are capable of undergoing subsequent differentiation after being cultivated in vitro. In this category are cells of the skin, cartilage, muscle, tendon, ligament, bone, adipose tissue, endothelium, and many others. Aside from skin, the foregoing cell types are harbored as stem cell populations in the marrow, in addition to those of the hematopoietic and immune systems, but the diversity of mesenchymal and possibly other cell types in the marrow still needs to be probed. Stimulating factors, the cytokines,which move some of the cells into the circulation, will be important for engineering acellular scaffolds. Other stem cells are available to tissue engineering, such as the satellite cells found in striated muscle and to some degree keratinocytes of the skin. Where host cells are available, an acellular scaffold, particularly one enhanced with signals and possessing the binding sites needed for cell attachment, can mobilize host cells that will populate the prosthesis.