Fig. 1.

The timeline of major scientific advances during the history of stem cell research. Multipotent stem cells were first discovered in 1961, representing the initial breakthrough in stem cell and regenerative medicine. Dolly the sheep was cloned in 1997. The transition from fundamental research, to pre-clinical research, and finally to clinical trials is driven by many discoveries and milestones. Many advances in reprogramming factor combinations, experimental methods, and the elucidation of signaling pathways have recently contributed to the first clinical trials for retinal cell transplants and spinal cord transplants. Red shading represents fundamental research, yellow shading represents pre-clinical work, and green shading represents clinical trials.