osteon

**Structure** - **Haversian (central) canal**: runs longitudinally; contains capillaries, venules, nerve fibers, and loose connective tissue (lined by endosteum). - **Concentric lamellae**: layers of type I collagen and hydroxyapatite arranged in rings around the canal. Collagen fiber orientation alternates between adjacent lamellae, which gives resistance to torsional and bending forces. - **Lacunae**: small spaces between lamellae that house osteocytes (mature, terminally differentiated bone cells). - **Canaliculi**: tiny channels connecting lacunae to each other and to the Haversian canal. Osteocyte processes and gap junctions in them allow nutrient/waste exchange and mechanosensation. - **Cement line**: the boundary of each osteon, a hypermineralized, collagen-poor layer that marks where resorption stopped and formation began. - **Volkmann (perforating) canals**: run perpendicular/oblique to Haversian canals. They connect adjacent osteons to each other and to the periosteum and medullary cavity, carrying vessels transversely. - **Interstitial lamellae**: remnants of old osteons between intact osteons, left over from prior remodeling. - **Circumferential lamellae**: outer (beneath the periosteum) and inner (lining the endosteum) layers that encircle the whole shaft.

**Development and remodeling** - Primary (woven) bone is replaced by secondary lamellar bone organized into osteons. - Remodeling occurs in basic multicellular units: an osteoclast "cutting cone" tunnels through bone, followed by osteoblasts that lay down new lamellae centripetally (from the periphery toward the center), which narrows the Haversian canal. - Osteoclasts: derived from monocyte/macrophage lineage; activated by RANKL (from osteoblasts/stromal cells) binding RANK. Osteoprotegerin (OPG) is a decoy receptor that inhibits this. Denosumab is a RANKL inhibitor. - Osteoblasts: derived from mesenchymal stem cells; produce osteoid, alkaline phosphatase, osteocalcin. - Parathyroid hormone (intermittent) is anabolic; continuous PTH elevation is catabolic.

**Clinical significance / high-yield associations** - **Osteoporosis**: reduced bone mass with normal mineralization and normal lab values (Ca, PO4, PTH, ALP). Resorption exceeds formation. Risk: postmenopausal estrogen loss, glucocorticoids, and age. Fractures: vertebral compression, distal radius (Colles), femoral neck. Treat with bisphosphonates (inhibit osteoclasts; risk of osteonecrosis of the jaw and atypical femoral fractures), denosumab, teriparatide, and raloxifene. - **Osteomalacia/rickets**: defective mineralization of osteoid (vitamin D deficiency) with low/normal Ca, low PO4, and elevated ALP and PTH. - **Paget disease of bone**: disordered, excessive osteoclast then osteoblast activity produces a chaotic "mosaic" pattern of lamellar bone with prominent cement lines. Features: elevated ALP with normal Ca/PO4, hat size increase, hearing loss, bone pain, and risk of high-output heart failure and osteosarcoma. - **Osteopetrosis**: osteoclast dysfunction (carbonic anhydrase II or other defect) causes dense but brittle bone, "Erlenmeyer flask" deformity, pancytopenia from marrow encroachment, and cranial nerve compression. - **Hyperparathyroidism**: osteitis fibrosa cystica (subperiosteal resorption, brown tumors). - **Osteomyelitis/osteonecrosis**: disrupted blood supply through Haversian/Volkmann canals leads to bone death (sequestrum). Staph aureus is the most common cause of osteomyelitis; Salmonella in sickle cell disease. - **Fracture healing**: hematoma, soft callus (fibrocartilage), hard callus (woven bone), then remodeling into lamellar osteons.

**Histology pearls** - Osteons are seen in cross-section as concentric rings ("tree-ring" appearance) on ground bone or decalcified H&E sections. - Compact bone has osteons; **cancellous (trabecular/spongy) bone** has trabeculae composed of **hemiosteons** (packets) instead of complete osteons and has no central Haversian canals (nutrients diffuse from marrow). - Woven bone (disorganized collagen, seen in fracture callus, Paget disease, and developing bone) is distinguished from lamellar bone (organized, found in osteons). - Cortical bone makes up ~80% of skeletal mass; trabecular bone has higher turnover and is affected first in osteoporosis (e.g., vertebrae).