Osteoarthritis, perhaps, was a disease of stem cell loss. The cells that were being worn out—in its first stages—were the cartilage-making stem cells, and they could no longer keep up the genesis of cartilage. The balance between growth and degeneration had been disrupted. What the injury had thrown off was the capacity of cartilage at the joint to maintain its internal balance—between the growth of new cartilage (via the stem cells) and the decay of old cartilage (via age and injury).
