while immobilization and surgery may facilitate healing, a fracture ultimately heals through physiological processes. the healing process is mainly determined by the periosteum (the connective tissue membrane covering the bone). the periosteum is one source of precursor cells which develop into chondroblasts and osteoblasts that are essential to the healing of bone. other sources of precursor cells are the bone marrow (when present), endosteum, small blood vessels, and fibroblasts.