One day Prof. Eldad Tzahor peered into a microscope in his lab and saw steak. As part of Tzahor's research into repairing muscle tissue, Dr. Tamar Eigler, a postdoctoral fellow in his lab at the Weizmann Institute of Science, had been experimenting with cultured muscle stem cells. One of these experiments had produced the surprising sight that appeared before Tzahor's eyes: The cells had started fusing into tiny fibers that thickened rapidly, within hours creating large muscle fibers resembling those in whole cut meat.