BONE ALVEOLI, SEM. BONE ALVEOLI, SEM The human skeleton is made of bone composed of calcium, an important element in body functions. The surface of bones are covered with a connective membrane called the periosteum. Below the periosteum the bone is dense like ivory (cortical bone) and very resistant. Deep in the bone is a layer of cancellous (or spongious) bone harboring the bone marrow. There are three types of bone cells: osteoblasts and osteocytes that construct bony tissue, and osteoclast