Osteon

Each osteon is a column ≈200–300 µm in diameter and a few millimetres long, running parallel to the long axis of the bone. It contains 4–20 concentric lamellae around a central (Haversian) canal (≈50 µm) that carries one or two capillaries or venules, nerve fibres and loose connective tissue lined by endosteum. Collagen fibres in successive lamellae spiral in alternating directions, giving the osteon great resistance to torsion.

Osteocytes occupy lacunae between lamellae; their processes run in canaliculi that radiate across the lamellae, connecting the cells to each other and to the central canal so that nutrients, wastes and signals can pass from cell to cell by gap junctions and through the pericellular fluid. Central canals communicate with one another and with the periosteum and marrow cavity via perforating (Volkmann) canals. The outer boundary of each secondary osteon is a cement (reversal) line, marking the limit of the resorption tunnel from which it was formed.

Osteons are continually created by remodelling: a cutting cone of osteoclasts bores a tunnel, then osteoblasts refill it from the outside in with successive lamellae, leaving a narrowed central canal. Partly resorbed old osteons persist as interstitial lamellae.