Compact bone. Light micrograph of a section through a mature human bone. Compact bone is a dense form of bone comprising units called osteons or Haversian systems (circular), which have a central cavity (black) containing blood vessels. Each osteon is made up of many bone cells called osteocytes. These have delicate, branching processes that form gap junctions with adjacent cells. Photographed with darkfield illumination. Magnification: x185 when printed 10cm wide.