the periosteum consists of dense irregular connective tissue. it is divided into an outer "fibrous layer" and inner "cambium layer" (or "osteogenic layer"). the fibrous layer contains fibroblasts, while the cambium layer contains progenitor cells that develop into osteoblasts. these osteoblasts are responsible for increasing the width of a long bone and the overall size of the other bone types. after a bone fracture, the progenitor cells develop into osteoblasts and chondroblasts, which are essential to the healing process.