There are two types of bone tissue: compact bone which is dense and forms the outer layer, and spongy bone which is less dense and found in low stress areas. Bones are composed of osteocytes embedded in bone matrix, as well as osteoblasts which build bone, osteoclasts which resorb bone, and osteogenic cells which develop into osteoblasts. A long bone has a central shaft made of compact bone surrounding marrow, expanded ends made of inner spongy and outer compact bone lined with marrow, and growth plates between the shaft and ends.