A research team led by Dr. Samir El-Dahr, Jane B. Aron Professor and chair of pediatrics at the School of Medicine, examined why human kidneys, which are composed of nearly a million filter units, stop creating new filter cells after birth. The researchers used a mouse model to understand what occurs when a fetal stem cell differentiates into a mature kidney cell. Researchers found that near the time of birth, the DNA blueprint that controls the fate of kidney stem cells changes dramatically.