Bone structural unit. Computer artwork of an osteon, the basic structural unit of compact bone. Compact bone is the outer layer of a bone shaft, and contains numerous osteons. Each osteon may have a blood vessel (here, an arterial capillary, red) and a nerve (yellow). The bone structure is continuously destroyed and rebuilt by bone cells. Osteoblasts (green) embed themselves on a bone surface and build new bone. Mature bone cells (osteocytes) are seen as the tangled, branching cells (brown) inside the bone tissue. Bone tissue is destroyed by osteoclasts (two seen here). One, at lower right, has been sectioned to reveal the acid (red) with which it destroys the bone.