A Haversian system, or osteon, measuring 100-500 micrometer in diameter and several centimeters in length, consists of 4 to 20 layers of Haversian lamellae arranged concentrically and a Haversian canal, which is located in the center of the system. Between lamellae are irregular oval lacunae occupied by osteocytes. They communicate via fine canaliculi. A Haversian canal is filled with loose connective tissue containing blood vessels, preosteoblasts, and macrophages. Haversian systems are demarcated by a clear line, the cement line, from each other or from interstitial lamellae.