Bone Growth and Development

Bone Growth and DevelopmentBone Composition & StructureMaterial Constituents:Calcium carbonate and Calcium phosphate60-70% bone weightAdds stiffnessPrimary determinant for compressive strength.CollagenAdds flexibilityContributes to tensile strength

2Long Bone StructureDiaphysis: shaft

Periosteum: fibrous connective tissue covering shaft

Epiphyses: the ends

Articular cartilage: hyaline cartilage covering epiphysis

Epiphyseal line & plate: the growth plate or the line left over in adult bones

Medullary Cavity: cavity in bone that contains marrow

Cellular Structure of Bone

The Haversian SystemHaversian Canal: Central Canal

surrounded by concentric layers of calcified matrix called lamellae.

Osteocytes are found between concentric lamellae and connected to each other, and the central canal by canals called canaliculi.

This network permits the exchange of nutrients and metabolic waste.

Cortical BoneCompact BoneLow porosity5-30% bone volume is non-mineralized tissueWithstand greater stress but less strain before fracturing

6Trabecular BoneSpongy BoneHigh porosity30 - >90% bone volume is non-mineralized tissueTrabeculae filled with marrow and fatWithstand more strain (but less stress) before fracturing7OssificationThe replacement of other tissues with bone

Two Major Types of OssificationEndochondral and IntramembranousEndochondral OssificationMost Bones of the SkeletonBone develops from cartilageBlood vessels infiltrate cartilage, forming bonePrimary ossification begins at the center of the diaphysisSecondary centers form later at the epiphyses

Intramembranous OssificationSkull, Mandible and ClavicleBone develops directly not from cartilageDevelops from the inside outfrom spongy to dense boneTypes of Bone GrowthEpiphyseal and AppositionalEpiphyseal GrowthVirtually all growth in length occurs at the growth platesGrowth plates fuse after puberty,as rate of ossification exceedes the growth of cartilage.Once the growth plate is entirely ossified (fused) there can be no further growth in bone lengthThis limits the size of adults

Appositional GrowthGrowth in circumfrenceThe inner layer of the periosteum builds concentric layers of bone

Combined Activity of Osteoclasts and Osteoblasts Determine Bone DensityOsteoblasts build new boneOsteoclasts reabsorb boneBone Response to StressWolfs LawIndicates that bone strength increases and decreases as the functional forces on the bone increase and decrease.

Bone Modeling and RemodelingMechanical loading causes strainBone ModelingIf Strain > modeling threshold, then bone modeling occurs.18Bone Response to StressBone mineral density generally parallels body weightBody weight provides most constant mechanical stressDetermined by stresses that produce strain on skeletonThink: weight gain or loss and its effect on bone density19Bone HypertrophyAn increase in bone mass due to predominance of osteoblast activity.Seen in response to regular physical activityEx: tennis players have muscular and bone hypertrophy in playing arm.The greater the habitual load, the more mineralization of the bone.Also relates to amount of impact of activity/sport20Bone AtrophyA decrease in bone mass resulting form a predominance of osteoclast activityAccomplished via remodelingDecreases in:Bone calciumBone weight and strengthSeen in bed-ridden patients, sedentary elderly, and astronauts21SummaryBone is an important living tissue that is continuously being remodeled.Bone Strength and Resistance to fracture depend on its material composition and organizational structure.Bones continually change in density.

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