The structural unit of bone is osteon. It is also called as Haversian system. Each osteon is a cylindrical unit that has several different parts. The central part of Haversian system is called as Haversian canal. Blood vessels pass through it. It is surrounded by concentric layers of lamellae. The lamellae hold osteoblasts together. Osteoblasts later become mature osteocytes. The osteocytes are embedded in bone.