Bone Histology Part 2: Mechanical Properties

Study Notes

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.

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.

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.

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.

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.

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.