fibers of a biocompatible , biodegradable or non - biodegradable , synthetic polymer are provided , and are formed into a three - dimensional scaffold . the fibers of the scaffold may have a branched configuration extending outwardly from a central stem . the fibers provide sufficient surface area to permit attachment to the scaffold in vitro of an amount of cells effective to produce functional vascularized organ tissue in vivo . fibers of the scaffold are spaced apart such that the maximum distance over which diffusion of nutrients and gases must occur through a mass of cells attached to the fibers is between 200 and 300 microns . the diffusion provides free exchange of nutrients , gases and waste to and from cells proliferating throughout the scaffold in an amount effective to maintain cell viability throughout the scaffold in the absence of vascularization . cells derived from vascularized organ tissue are attached in vitro to the surface of the fibers uniformly throughout the scaffold and the cells are grown on the scaffold in a nutrient solution in vitro to form a cell - scaffold composition . growth factors , compounds stimulating angiogenesis and immunomodulators may be provided in the cell - scaffold composition and the fibers may have a coating to enhance cell attachment . the cell - scaffold composition is implanted in a host at a location having adequate vascularization to allow growth of blood vessels into the cell - scaffold composition . combinations of cell - scaffold compositions containing different cell populations may be implanted .