Haversian canal

**Structure** - Osteon (Haversian system) = the functional unit of compact bone: a central Haversian canal surrounded by concentric lamellae of mineralized matrix (mostly type I collagen + hydroxyapatite). - Canal contents: capillaries/small arterioles and venules, unmyelinated and myelinated nerve fibers, lymphatics (limited), and loose connective tissue with osteoprogenitor cells. - Osteocytes sit in lacunae between lamellae and communicate via canaliculi (cytoplasmic processes joined by gap junctions), allowing nutrient/waste exchange with the Haversian canal vessels. - Canal is lined by endosteum (osteoprogenitor cells, bone-lining cells).

**Related canals/vessels** - Volkmann (perforating) canals: run perpendicular/oblique to the long axis; connect Haversian canals to each other, the periosteum, and the medullary cavity. Lamellae are not concentric around them. - Interstitial lamellae: remnants of old osteons between newer ones. - Circumferential lamellae: outer (beneath periosteum) and inner (lining marrow cavity). - Cement line: boundary of each osteon, marks the limit of resorption before new bone formation.

**Remodeling (clinically relevant)** - Cortical bone remodels via a "cutting cone": osteoclasts tunnel through bone (forming a new Haversian canal), followed by osteoblasts laying down concentric lamellae (closing cone) until only the central canal remains. - Coupling is regulated by RANKL/OPG, PTH, vitamin D, calcitonin, estrogen, and mechanical stress (Wolff's law). - Osteoblasts: make osteoid (type I collagen), express ALP, osteocalcin, RANKL. Osteoclasts: derived from monocyte/macrophage lineage, multinucleated, acid phosphatase (TRAP)-positive, ruffled border; activated by RANKL via RANK.

**Clinical correlations** - Osteoporosis: reduced bone mass with normal mineralization; widened Haversian canals/increased cortical porosity; fracture risk (vertebral compression, femoral neck, distal radius/Colles). - Paget disease of bone: disordered, excessive remodeling; "mosaic" pattern of lamellar bone; ↑ ALP with normal Ca/Phos; hat size increase, hearing loss, high-output heart failure, risk of osteosarcoma. - Osteomyelitis: infection spreads through Haversian and Volkmann canals; may cause sequestrum (necrotic bone) and involucrum (new periosteal bone). - Osteopetrosis: osteoclast dysfunction (carbonic anhydrase II or CLCN7 defects) causing dense, brittle bone, marrow obliteration, cranial nerve compression, pancytopenia. - Hyperparathyroidism: ↑ osteoclastic resorption (subperiosteal resorption, osteitis fibrosa cystica, brown tumors). - Fracture healing: Haversian systems re-establish across the fracture during remodeling of woven bone to lamellar bone.

**Board pearls** - Compact bone = Haversian systems; cancellous (trabecular) bone = trabeculae without Haversian canals (nutrients by diffusion from marrow spaces). - Haversian = longitudinal; Volkmann = transverse. - Nutrient artery supplies the inner two-thirds of cortex; periosteal vessels supply the outer third. - Do not confuse "Haversian" (bone) with "Haversian glands" (old term for synovial fat pads).