Embryonic stem cells come from donated excess fertilized eggs created for the in vitro fertilization (IVF) process, tissue that would otherwise be discarded. Induced pluripotent stem cells are generated in a lab, often by manipulating “mature” cells (such as skin or blood cells) to an embryonic-like state through a process called “reprogramming.” Tissue stem cells are derived from their “home” tissue; for example, muscle stem cells from muscle tissue.