Haversian system

STRUCTURE - Haversian canal: central longitudinal channel (~50 µm) containing capillaries/venules, nerve fibers, and loose connective tissue. Lined by endosteum. - Concentric lamellae: layers of mineralized matrix (type I collagen + hydroxyapatite). Collagen fiber orientation alternates between adjacent lamellae, giving resistance to torsion and bending. - Lacunae: small spaces between lamellae that house osteocytes. - Canaliculi: tiny channels radiating from lacunae. Osteocyte processes (connected by gap junctions) pass through them, allowing nutrient/waste exchange and mechanosensation. - Cement line: boundary of the osteon that marks where bone resorption stopped and new formation began. - Interstitial lamellae: remnants of old osteons between intact osteons. - Circumferential lamellae: outer (beneath periosteum) and inner (beneath endosteum) layers encircling the whole shaft. - Volkmann (perforating) canals: run perpendicular/oblique to Haversian canals. They connect Haversian canals with each other, the periosteum, and the marrow cavity. They are not surrounded by concentric lamellae.

DEVELOPMENT AND REMODELING - Osteons form through remodeling of existing bone, in a basic multicellular unit (BMU). Osteoclasts form a "cutting cone" that tunnels through bone. Osteoblasts follow in the "closing cone" and lay down new concentric lamellae (osteoid, then mineralization). Osteoblasts that become trapped in matrix become osteocytes. - Osteoclast activity is regulated by RANKL/RANK/OPG. PTH, vitamin D, calcitonin, mechanical stress (Wolff's law), and estrogen influence remodeling. - Compact bone is lamellar and organized into osteons. Woven bone (seen in fracture callus, Paget disease, and the developing fetal skeleton) is disorganized and lacks osteons. It is later remodeled into lamellar bone. - Cortical bone makes up ~80% of skeletal mass and is found in diaphyses of long bones. Cancellous (trabecular) bone has trabeculae rather than osteons (sometimes called packets), and is more metabolically active.

CLINICAL SIGNIFICANCE - Osteoporosis: decreased bone mass with normal mineralization, leading to cortical thinning and trabecular loss. Lab values (Ca, PO4, ALP, PTH) are normal. - Osteomalacia/rickets: defective mineralization of osteoid (low vitamin D). - Paget disease of bone: excessive, disorganized remodeling with a "mosaic" pattern of lamellar bone; elevated ALP with normal Ca/PO4. Complications include high-output heart failure, hearing loss, and osteosarcoma. - Osteopetrosis: defective osteoclasts (carbonic anhydrase II or other defects) cause dense, brittle bone, with marrow failure and cranial nerve compression. - Osteonecrosis/avascular necrosis: loss of vascular supply through the Haversian/Volkmann systems leads to osteocyte death (e.g., femoral head after femoral neck fracture, steroids). - Osteomyelitis: infection can spread via Volkmann canals to the subperiosteal space, and can cause sequestrum formation (dead bone) and involucrum (new bone). - Fracture healing: soft callus → hard callus (woven bone) → remodeling into lamellar bone with new osteons.

HIGH-YIELD PEARLS - Haversian system = osteon = structural unit of compact bone. - Haversian canals run longitudinally; Volkmann canals run transversely. - Osteocytes are the most abundant bone cells, and they sense mechanical strain. - Named after Clopton Havers (17th-century English physician).