Abstract: A matrix structure containing attached cells such as endocrine cells, fibroblasts, endothelial cells or genitourinary cells is implanted in a patient adjacent tissue having a high surface area and vasculature such as mesentery, omentum or peritoneum tissue. Large volumes of cells can be attached to the matrix and the matrix implanted with minimum trauma and blood loss into a patient to produce a functional organ equivalent. Multiple matrix structures containing cells can be implanted to functionally resemble naturally occurring organs. Implanting multiple matrices between folds of the mesentery is particularly well suited for growth of endocrine structures, including liver, pancreas, and adrenal gland. The matrix structure is preferably formed from a biodegradable artificial polymer. Collagen and non-biodegradable materials can also be used, and the matrix structure can be overlaid with a material that enhances cell attachment.