Figure 1: Tissue engineering triad of cells or source, signals (provided chemically by growth factors/cytokines or physically by a bioreactor), and the scaffold which acts as a template for tissue formation allowing the cells to migrate, adhere, and produce tissue. These components make up the Triple S (Source, Scaffold, and Signal) of tissue engineering. Any combination of these triad components has been considered in various studies in cell therapy which accounts for SS (Scaffold/Signal, Scaffold/Source, and Signal/Source). Finally, these three components (Source, Scaffold, and Signal or SSS) have also been considered altogether in some research projects.