Bone Histology

Expansion

Cells

• Osteoblasts: build bone, secreting osteoid; express RANK ligand
• Osteocytes: mature cells trapped in lacunae, communicating through canaliculi; act as the mechanosensors
• Osteoclasts: multinucleate, from monocyte lineage, resorbing bone in Howship lacunae; carry RANK
• Bone lining cells

Structure

• Compact (cortical) bone: osteons (Haversian systems) of concentric lamellae around a central Haversian canal carrying vessels, connected by Volkmann canals
• Cancellous (trabecular) bone: trabeculae aligned along stress lines, with marrow between

Woven versus lamellar

• Woven: randomly arranged collagen, laid down rapidly. Normal in the fetus and in fracture callus; always pathological in adult bone otherwise, and seen in Paget disease, fibrous dysplasia and osteosarcoma
• Lamellar: organised, mature, strong

Ossification occurs by two routes: intramembranous (flat skull bones, clavicle, directly from mesenchyme) and endochondral (long bones, replacing a cartilage model).