10.5:
Compact Bone
Compact or cortical bone is a hard, solid osseous tissue found in the cortex of all bones. It also forms the bulk of the diaphysis in long bones.
The structural unit of compact bone is the osteon, also called the haversian system. They are long, cylindrical units that are arranged in parallel to withstand compression forces along the long axis.
Each osteon contains concentric lamellae surrounding a central osteonic or haversian canal. The collagen fibers of each lamellar ring are oriented alternately, thus resisting twisting forces.
Between the lamellae are spaces called lacunae, which harbor osteocytes — the mature bone cells.
The haversian canal encloses blood vessels, lymphatic vessels, and nerves connected to the osteocytes via tiny canals called canaliculi.
The transverse Volkmann's canal connect the osteons to the main blood and nerve supply to the bone.
Interstitial lamellae are found between the osteons. They are incomplete rings of bone matrix that are the remnants of older osteons being recycled via bone remodeling.
The circumferential lamellae surround the osteons and are present around the entire circumference of the bone.
10.5:
Compact Bone
Most bones contain compact and spongy osseous tissue, but their distribution and concentration vary based on the bone's overall function.
Compact bone, also called cortical bone, is the denser, stronger of the two types of bone tissue. It is found under the periosteum and in the diaphyses of long bones, where it provides support and protection. The microscopic structural unit of compact bone is called an osteon, or haversian system. Each osteon is composed of concentric rings of calcified matrix called lamellae. Running down the center of each osteon is the central canal, or haversian canal, which contains blood vessels, nerves, and lymphatic vessels.
Mature bone cells (osteocytes) are located inside spaces called lacunae within the lamellar rings. The lacunae are interconnected via narrow, branched canals called canaliculi. These canaliculi house the long cellular processes of adjacent osteocytes that connect with each other via gap junctions. The canaliculi network eventually connects with the central canal, allowing nutrient and waste exchange through blood.
Stress Resistance
The concentric lamellae of osteons are made of inorganic crystals and collagen fibers embedded in an organic matrix. The direction of collagen fibers alternates between adjacent lamellae, making the structure highly resistant to twisting forces. The osteons are arranged in parallel in the compact bone. This arrangement helps resist compression forces along the long axis of the bone but not in other directions. Thus, long bones such as the femur can bear weight along the long axis, but fracture when sufficient force is applied perpendicular to its axis.
Part of this text is adapted from Openstax, Anatomy and Physiology 2e, Section 6.3: Bone structure