Bone Histology Part 2: Mechanical Properties

Configurations of Bone

• Cancellous bone (spongy or trabecular bone) contains scattered bone spicules and is found in developing bones, predominantly in the medullary cavity and the epiphysis.
• Dense bone (compact or cortical bone) is densely arranged osteons or layers of bone, found in the diaphysis of long bones.

Organization of Lamellar Bone

• Lamellar bone is organized into Haversian systems (osteons), which are organized osteocytes in concentric rings around a central canal.
• Non-Haversian systems are osteons that do not surround a central canal, and are also called interstitial bone.
• Circumferential bone is formed by adding layers of bone to the periosteum and endosteum by appositional growth.
• Outer circumferential bone is formed by the periosteum, while inner circumferential bone is formed by the endosteum.

Bone Remodeling and Regulation of Serum Calcium

• Decreases in calcium levels trigger osteolysis, where osteocytes extract calcium from surrounding matrix without structural changes.
• Long-term high demand for calcium in hypocalcemia leads to osteoclasts removing entire osteons, creating resorption spaces.

Mechanical Properties of Bone

• Tensile properties of bone relate to collagen, while compressive properties relate to minerals.
• Compressive properties of bone are twice as strong as tensile properties, making tensile fractures more common than compression fractures.

Homogenous Cellularity

• Mesenchymal cells differentiate into osteoprogenitor cells, osteocytes, and osteoblasts.
• Osteoblasts secrete osteoid and appear columnar in shape when active, and flat when inactive.
• Osteocytes occupy lacunae and are interconnected with osteoblasts through gap junctions for communication and ion flux.
• Osteoclasts are fused macrophages that dissolve and phagocytize bone, removing matrix and cellular components.

Types of Bone

• Woven bone is an immature or fibrous bone that is easily sectioned, has randomly distributed lacunae, and is found in developing bones, fracture sites, and bone cancer (osteosarcoma).
• Lamellar bone is a mature bone that is highly organized into layers containing lacunae, and is organized around osteonal (Haversian) canals that contain blood vessels.