Expansion
Compact bone in osteons, cancellous bone in trabeculae
Expansion
Two architectures
- Compact (cortical) bone - dense, forming the shaft. Organised into osteons (Haversian systems): concentric lamellae around a Haversian canal carrying vessels and nerves, linked by Volkmann’s canals
- Cancellous (trabecular, spongy) bone - a lattice of trabeculae aligned along lines of stress, filled with marrow. Metabolically far more active, so it is lost first in osteoporosis, which is why vertebral and femoral neck fractures come early
Two tissue types
- Lamellar bone - organised parallel collagen, strong. All mature bone
- Woven bone - disorganised collagen, weak. Normal only in the fetus and in fracture callus; elsewhere it indicates Paget’s disease, fibrous dysplasia, osteosarcoma or hyperparathyroidism
Cells
- Osteoblasts - build bone, express alkaline phosphatase and RANK ligand
- Osteocytes - trapped osteoblasts in lacunae, communicating through canaliculi, acting as mechanosensors
- Osteoclasts - multinucleate, from the monocyte lineage, resorbing bone via a ruffled border and acid secretion. Inhibited by bisphosphonates and denosumab
Osteocytes sense loading, which is why bone follows Wolff’s law, remodelling along lines of stress, and why immobility causes rapid bone loss.