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Structure and Composition of BoneBiomechanics aims to explain the mechanics of life and living. From molecules to organisms, everything must obey the laws of mechanics.Y. C. Fung

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Learning ObjectivesIdentify important structures of bone at gross, macro and micro levels.Compare contributions of different components of bone (such as cells) to its functionsDescribe the effects of structure of bone on its functions (Wolffs Law)

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Bone a quick overviewComposite of extracellular matrix proteinsMineralizes through elegant feedback and regulation mechanisms Cellular components Maintain structural integrityRespond to metabolic and mechanical requirementsFunctionsProtects organsSupportsAllows movement through attachment for musclesManufactures blood cells (bone marrow)Stores Ca2+ and PO43-

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Bones can be classified on the macro level several ways, such as by position...Axial skeleton Bone forming axis of body, e.g skull, rib, sternum and vertebrae

Appendicular skeleton bones forming appendages of body, e.g. limbs, shoulder and hip.

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Long Bones - longer than they are wide (e.g humerus)

Short bones - cube shaped, e.g. bones in wrist and ankle

Flat bones - Thin, flattened and a bit curved (e.g sternum and most skull bones)

Irregular bones - Complicated shapes (e.g vertebrae, maxilla and mandible)

or by shape

Long bones: Long shaft with two bulky ends or extremities; Primarily compact bone but may have spongy bone at ends or extremitiesShort, flat and irregular bones: Mainly spongy bone covered by a thin layer of compact bone; Short bones can be those that form within tendons (e.g. patella)

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or by structure.Compact (cortical) boneSpongy (cancellous, trabecular) bone

Bones can also be classified by how they develop (from the fetal stage through bone growth into adulthood), including membranous bone (ectodermal) and cartilaginous (endochondral) bone. But that is beyond the scope of this course.

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Structure of long bonesDiaphysisEpiphysisMetaphysisArticular cartilagePeriosteumMedullary canal (marrow cavity)Endosteum

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Structure of short, irregular and flat bonesThin plates of periosteum-covered compact bone on the outside Endosteum-covered spongy bone on the insideNo diaphysis or epiphysisBone marrow between trabeculae

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Microstructure of adult bone: Lamellar boneAdult bone made up of layers (lamellae)Bundles of collagen in parallel surrounded by mineralized matrixBetween layers are lacunae (spaces)Lacunae contain osteocytes (bone cells)Canaliculi (fine canals) spread from lacunae

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Bone cells are never more than 100 m from a blood supply

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Interstitial lamellaeConcentric lamellae

Compact Bone:Lamellae arranged in concentric rings around Haversian canals containing blood, lymph and nerves Perforating (Volkmanns) canals connect blood, lymph and nerves from periosteum with medullary canal and Haversian canalsOsteon (Haversian system) = one Haversian canal and lamellae around it; typically about 200 microns, so no point is more than 100 microns from the central blood supplyInterstitial lamellae are like partial osteons, and are no more than 100 microns from a blood supply.

Spongy Bone:No osteonsTrabeculae - Lamellae arranged in latticework of thin plates of bone Spaces between trabeculae filled with red marrow which produces blood cells

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Hierarchical structure of bone

Section A: ~10 mmSection B: ~200 micronsSection C: ~ 20 microns; collagen fibrils are ~200 nm12

Composition of bone~1/3 organic Collagen fiber matrix (~88%) organized around lamellaeNon-collagen (~12%); Glycoproteins, proteoglycans, lipids, etc)~1/3 inorganic (mineral) Mostly dense calcium phosphate in the form of highly insoluble crystals of hydroxyapatite~1/3 waterCells osteoblasts, osteoclasts, osteocytes, bone lining cells

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Bone cells: OsteoblastsBone formers: produce organic matrixFound on surfaces where bone is formedResponsible for growth in thickness of long bonesCan initiate bone resorptionSynthesize osteoid, collagenControl mineralizationExtend to adjacent osteoblasts through canaliculiReceptors estrogen, parathyroid hormone, Vitamin D, growth factors

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Bone cells: OsteocytesEntrapped osteoblastsMost numerous bone cellDerived from osteoblast that is no longer synthesizing collagenFunction: Maintenance of bone

Osteocytes synthesize collagen, control mineralization within lacunae, may be involved with resorptionThey maintain calcium exchange between bone and extracellular fluid

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Bone cells: OsteoclastsLarge multinucleated cellPlays active role in bone resorption during bone remodellingPrecursors: monocytes from marrow or bloodFound in notches or indentations in bone (Howships lacunae) excavationsContain the enzyme acid phosphataseReceptors: calcitonin, estrogen (indirect)

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Bone Lining CellsCover inactive (nonremodeling) bone surfacesConnect to other cells through canaliculiMay play important roles inHematopoiesisMechanotransduction propagating signals initiating bone resorption and remodelingMaintenance of bone fluids and movement of ions for mineral homeostasis

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Relationships between structure and functionWolffs law: change in form follows change in function Bone structure changes in response to mechanical stimulation

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Relationships between structure and functionMechanotransduction: Mechanical signals Biochemical signalsResponse: cell activity Bone formation/resorption depends on duration, magnitude and rate of loadingCyclic loading induces bone formationUnloading induces bone resorption

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