A human kidney is built exactly once. Everyone is born with somewhere between 200,000 and 2 million nephrons, the microscopic filtering units that clean the blood, and the body never makes another one. Nearly everything known about how that construction works has been inferred from mice.
Researchers at the Penn-CHOP Kidney Innovation Center have now read instructions directly from human tissue, and one of their findings contradicts a long-standing assumption. Cells that had already started down the path toward becoming one kind of kidney cell were caught turning into a different one.
