A cell-scaffold composition is prepared in vitro for implanting to produce functional organ tissue in vivo. The scaffold is three-dimensional and is composed of hollow or solid fibers of a biocompatible, synthetic polymer which is biodegradable or non-biodegradable. The fibers of the scaffold may have a branched configuration extending outwardly from a central stem. 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 in an amount effective to produce functional vascularized organ tissue in vivo, the cells are grown on the scaffold in a nutrient solution in vitro to form the cell-scaffold composition which is implanted in a host at a location having adequate vascularization to allow growth of blood vessels into the 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. Combinations of cell-scaffold compositions containing different cell populations may be implanted.