Schematic depicting the principles of tissue engineering. (A), (B) Cells are generally expanded from an autologous or an allogeneic source. (C) A scaffold is used to support cell growth in the presence of specific growth factors and mechanical stimuli. 3D, three-dimensional. (D) The combination of scaffold, cells, growth factors, and mechanical stimuli recreates a functional microenvironment that stimulates tissue organization into an engineered graft, which is then transplanted into a patient.