A drop in serine levels tells hair follicle stem cells to stop growing hair and start repairing wounds. This stress-driven switch could open new paths to accelerate skin healing through diet or medication. Credit: Shutterstock The skin contains two main types of adult stem cells: epidermal stem cells and hair follicle stem cells. Normally, each type has a clear role -- one maintains the skin while the other supports hair growth. However, research from Rockefeller University has revealed that