Stem cells: The best studied progenitor cells are those in the bone marrow, which give rise to all blood cell types and certain cells of the immune system. More recently discovered is "brain marrow," which consists of collections of neural stem cells that line spaces in the brain. Under certain conditions, they can give rise to new nervous tissue. Researchers are currently identifying sources of stem cells in all tissues, and hope to use them to grow new tissues in an approach called regenerative medicine. Replacement neural tissue, for example, is being developed to treat spinal cord injuries and neurodegenerative disorders. The skeleton is particularly vulnerable to injury during the turbulent teen years, when bones grow rapidly.  Athletic teens sometimes develop Osgood-Schlatter disease, which is a painful swelling of a bony projection of the tibia below the knee.  Overusing the thigh muscles to straighten the lower limb irritates the area, causing the swelling.  Usually a few months of rest and no athletic activity allows the bone to heal on its own.  Rarely, a cast must be used to immobilize the knee.