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