Bone Composition and Organization

Classification: Skeletal

**Figure 1** Diagram of a cross section of bone, depicting the structural differences between compact and spongy bone.

Bone Matrix

Bone matrix has both organic (30%) and inorganic components (70%)

Organic components

Main function: soft, resisting tension
Cells: osteoclasts, osteoblasts, osteocytes
Osteoid: ground substance (proteoglycans, T1 collagen, CT)
Trace minerals: F-, Cl-, K+, Mg2+

Inorganic components:

Main function: hard, resisting compression
Hydroxyapatite crystals (Ca₁₀(PO₄)₆(OH)₂)

Compact & Spongy Bone

Compact bone organization (aka cortical bone)

Osteons: cylindric structures placed concentrically around each other

Lamellae

Concentric lamellae: concentric layers of osteons
Interstitial lamellae: between concentric lamellae, fill in gaps or are derivatives of old osteons
Circumferential lamellae: lamellae around the order border of bones (beneath the periosteum)

Why lamellae? The fibers of each lamellae are parallel, but fibers of adjacent lamellae are alternating, thus providing an additional later of torsional resistance

Canals

Central (Haversian) canal: within each osteon
Perforating (Volkmann’s) canal: perpendicular to central canal, connect vascularization of compact bone to medullary cavity/spongy bone

Lacunae: small cavities at lamellar junctions where osteocytes reside and sense bone matrix composition

Canaliculi

Passages between lacunae filled with fluid; facilitate communication between osteocytes in an osteon and regulate the transportation nutrients/waste
How do canaliculi form? They are formed from the remnants of osteoblast cytoplasmic extensions who got trapped in osteoid

Spongy bone organization (aka cancellous bone)

No osteons

Trabeculae: porous network, contains irregular lamellae with osteocytes