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…
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.
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Q1: What is an osteon and what role does it play in compact bone structure?
An osteon, also called a haversian system, is the structural unit of compact bone. It consists of concentric lamellae surrounding a central haversian canal containing blood vessels, nerves, and lymphatic vessels. Osteons are arranged in parallel to withstand compression forces along the long axis of bones, providing strength and support.
Q2: How do osteocytes receive nutrients within compact bone?
Osteocytes reside in spaces called lacunae within the lamellae of osteons. Tiny canals called canaliculi connect lacunae to the central haversian canal, allowing osteocytes to exchange nutrients and waste through blood vessels. This network ensures mature bone cells remain viable despite being embedded in solid matrix.
Q3: Why are collagen fibers arranged alternately in compact bone lamellae?
Collagen fibers in adjacent lamellae are oriented in alternating directions, creating a highly organized structure that resists twisting forces. This alternating fiber arrangement, combined with the parallel orientation of osteons, allows compact bone to withstand both compression and torsional stresses effectively.
Q4: What are interstitial lamellae and how do they form?
Interstitial lamellae are incomplete rings of bone matrix found between osteons. They represent remnants of older osteons that have been recycled during bone remodeling. These structures fill gaps left when the skeleton removes and replaces bone tissue, maintaining bone density and structural integrity.
Q5: How does the arrangement of osteons affect bone strength?
Osteons are arranged in parallel within compact bone, an orientation that resists compression forces along the bone's long axis. However, this parallel arrangement provides less resistance to perpendicular forces, which is why long bones like the femur can bear weight along their length but fracture when sufficient force is applied sideways.
Q6: What is the relationship between compact bone and the diaphysis of long bones?
Compact bone forms the bulk of the diaphysis, the shaft of long bones, and is found under the periosteum. This dense, solid osseous tissue provides the structural support and protection necessary for long bones to bear weight and resist mechanical stress during movement and activity.
Q7: What do Volkmann's canals do in compact bone?
Volkmann's canals are transverse canals that connect individual osteons to the main blood and nerve supply of the bone. These channels allow blood vessels and nerves to branch from the central supply and reach multiple osteons, ensuring all regions of compact bone receive adequate nutrition and nervous system communication.