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Bones andSkeletal Tissues

The SkeletonWhat are the components of the skeletal system?

Cartilage – offers support, resilience and flexibilityHyaline – nose, joint cavities Elastic – ear, epiglottisFibrocartilage – pubic symphysis & intervertebral discs

Bone – organs of the systemAlso contain nervous, connective and epithelial tissues

Ligaments – connect bone to boneTissue type?

Dense regular connective tissue

Membranes?

CartilageWhat is the importance in relation to the skeletal system?

Most bones develop from a cartilage modelProvides stability in some freely moving (synovial) jointsProvides shock absorption due to ability to compress & expand

Cartilage is covered with perichondriuma dense irregular connective tissue to preserve integrity of the cartilageAlso aids in growth and repair of the cartilage.

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Cartilages in the Adult Body

Growth of CartilageTwo mechanisms for growth:

Appositional growth Chondroblasts in surrounding perichondrium produce new cartilageCauses an increase in the width of the cartilage/bone model

Interstitial growth Chondrocytes within cartilage divide and secrete new matrixCauses an increase in the length of the cartilage/bone model

Function of BonesSupport

provides hard frameworkMovement/Leverage

skeletal muscles use bones as levers & along with the muscular system allow for movement of the body

Protection of underlying organsSoft tissues of the lungs, and spinal cord, the heart and organs within the pelvic cavity are well protected by bone

Storagereservoir for important mineralsenergy storage in the adipose of yellow marrow

Blood-cell formationbone contains red marrow

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Classification of BonesLong bones

longer than wide containing a shaft plus endsExample: femur

Short bonesroughly cube-shapedExample: carpals & tarsals

Sutural (Wormian) bonesSmall flat bones found in the sutures of the skull

Sesamoid bonesSmall, round and usually flattened slightly and develop inside of tendonsExample: patella

Flat bonesthin and flattened, usually curvedExample: cranial bones (frontal, parietal, temporal, occipital), ribs

Irregular bonesvarious shapes, do not fit into other categoriesExample: vertebrae

Bone Structure

Bones are composed of osseous tissueMatrixCells

Outer layer of tissue on bone is the periosteumInner layer of tissue in bone is the endosteum (lining the marrow cavity)

Microscopic Structure of BoneMatrix

FibersMainly collagen

Ground substancemineralized inorganic material called hydroxyapatite

Calcium Phosphate - Ca3(PO4)2 and Calcium Hydroxide -Ca(OH)2 are the main components that interact to form hydroxyapatite

Ca10(PO4)6(OH)2

Other substances (calcium carbonate, sodium, magnesium and flouride also become incorporated in the hydroxyapatite providing strength

Both are merged in osseous tissue the collagen provides a “framework” for the inorganic salts creating a tissue that is flexible but strong

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Microscopic Structure of Bone

CellsOsteocytes – the mature bone cell

Maintain the matrix by controlling calcium salt deposits in the matrix and the release of calcium into the bloodHoused in lacunae that are embedded between the layers of matrix (lamellae)Communicate via canaliculi, allowing osteocyte processes tocommunicate with adjacent osteocytes across by diffusion or via gap junctions

Microscopic Structure of BoneCells, cont.

OsteoblastsLocated on the inner and outer surface of boneSecrete osteoid (organic portion of matrix) which later becomes mineralizedResponsible for osteogenesisOnce surrounded by matrix it becomes an osteocyte

Osteoprogenitor Cells (progenitor = ancestor)The mesenchymal cells that differentiate into osteoblastsFound on the inner lining (endosteum) and the outer lining (periosteum)

OsteoclastsLarge cells that cause osteolysissecretion of acids that dissolve the matrix

Osteoclast

Maintenance of the Matrix

Matrix maintenance is a balance between osteoclast and osteoblast activity.

Osteoclast>osteoblast = bone removal (resporption)Osteoclast<osteoblast = bone addition (deposition)

Controlled by hormones that regulate blood Ca2+ levelsCalcitonin (CT)

reduces Ca2+ plasma levelsParathyroid Hormone (PTH)

Elevates Ca2+ plasma levels

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Compact vs Spongy BoneThe matrix may be highly organized or unorganized

Compact bone = organized and relatively solidSpongy bone = unorganized and open design

Both types:present in boneshave osteocytes, canaliculi and lamellae

Compact BoneFunctional unit is the Osteon (Haversian system)Concentric lamellae surrounding a central canal, collagen fibers spiral in different directions in each layer.Also forms larger circular rings called circumferential lamellaethat surround many osteons and the matrix that “fills in” the spaces around the osteons = interstitial lamellae

Transverse Section

Compact vs Spongy BoneSpongy Bone

Matrix organized into plates of parallel lamellaeForming a lattice called trabeculaeCanaliculi open to endosteum to gain nutrientsLight weight and high strength

Locations of compact & spongy bone:Spongy in the ends of bones and in the marrow or medullary cavitiesCompact is lining all bones, thickest in areas of high stresses

Microscopic Structure of Compact Bones

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Structure of a Typical Long BoneDiaphysis

Forms the shaft of a boneEpiphysis

Forms the ends of a boneBlood vessels

Osseous tissue is well vascularized with vessels in canals in compact bone and running through the trabecular spaces in spongy bone

Marrow or Medullary cavity hollow cavity – filled with marrow

MembranesPeriosteum – continuous with joint capsules and tendons

Isolates and protects the bone from surrounding tissueProvides location for nervous and vascular tissue to attach and enterInvolved in bone growth and repair (contains osteoprogenitor cells in the inner layer)

Sharpey’s fibers (extension of tendons through the periosteum)Endosteum – lines the marrow cavity

Structure of a Long Bone

Structure of Short, Irregular, and Flat Bones

Still contains compact bone and spongyNo medullary cavity

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Bone Design

Bone design and stress

Anatomy of a bone reflects stresses placed on itCompression and tension forces are largest at external surfaces

Macro to sub-nanostructure of bone

Bone DevelopmentOssification (osteogenesis) – bone-tissue formation

Membrane bones –formed directly from mesenchyme by the process of: Intramembranous ossificationClavicle, frontal, parietal occipital, temporal bones

Enchchondral bones –develop initially from hyaline cartilage by the process of Endochondral ossification

Ossification vs. CalcificationOssification is conversion of cartilage to boneCalcification is the addition of calcium salts to bone

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Intramembranous Ossification

Endochondral Ossification

Anatomy of Epiphyseal Growth AreasIn epiphyseal plates of growing bones

Cartilage is organized for quick, efficient growthCartilage cells form tall stacks divided into zones

Chondroblasts at the top of stacks divide quicklyPushes the epiphysis away from the diaphysisLengthens entire long bone

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Anatomy of Epiphyseal Growth Areas

Older chondrocytes signal surrounding matrix to calcifyOlder chondrocytes then die and disintegrate

Leaves long trabeculae (spicules) of calcified cartilage on diaphysis sideTrabeculae are partly eroded by osteoclastsOsteoblasts then cover trabeculae with bone tissueTrabeculae finally eaten away from their tips by osteoclasts

Growth of Other Types of Endochondral Bones

Short bones – arise from a single ossification centerIrregular bones – develop from distinct ossification centersSmall long bones

Form from a primary ossification center and a single secondary ossification center

Hormonal Regulation of Bone Growth

Growth hormoneproduced by the hypophysis (anterior pituitary gland)Stimulates epiphyseal plates

increases rate of division of chondrocytesThyroid hormone

ensures that the skeleton retains proper proportions

Sex hormonesPromote bone growth

Cartilage growth increases, butRate of osteoblast activity also increases, and

Synergistically with hGH and thyroid hormones induces closure of epiphyseal plates (between 18-25 yrs)

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Bone Remodeling

Bone deposit and removalOccurs at periosteal and endosteal surfaces

Bone remodeling Bone deposition – accomplished by osteoblastsBone reabsorption – accomplished by osteoclasts

Remodeling, Spongy Bone

Figure 6.12

Repair of Bone Fractures

Simple and compound fracturesSimple – breaks but does not penetrate skinCompound – breaks and protrudes through skin

Treatment by reductionClosed reduction – realignment by handOpen reduction – realignment by surgery

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Stages of Healing a Fracture

Figure 6.14

Common Types of Fractures

Table 6.1

Common Types of Fractures

Table 6.1

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Common Types of Fractures

Table 6.1

Osteoporosis – characterized by low bone massBone reabsorption outpaces bone depositionOccurs most of in women after menopause

Alcohol consumption in post-menopausal women has been shown to increase osteoblast activity.

Williams, F., et al. The effect of moderate alcohol consumption on bone mineral density: A study of female twins. Annals of the Rheumatic Diseases, 2004. Published Online First: 1 July 2004. doi:10.1136/ard.2004.022269; Moderate alcohol drinking helps prevent osteoporosis. Medical News Today, July 1, 2004; Innes, John. Moderate amounts of alcohol could protect against brittle bones. The Scotsman (Edinburgh, Scotland), July 1, 2004

Disorders of Bones

Disorders of Bones

Osteomalacia – occurs in adults –bones are inadequately mineralizedRickets – occurs in children – analogous to osteomalacia Paget's disease – characterized by excessive rate of bone deposition Osteosarcoma – a form of bone cancer