bone & cart lecture

8/6/2019 Bone & Cart Lecture

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CARTILAGE

&

BONE


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CARTILAGE & BONE

Skeletal system supporting / connective tissue

Functional differences relate to diff. nature &

proportion of ground substance & extracellular

matrix

BONE: rigid protective support / framework

CARTILAGE :semi-rigid (limited)

:precursor of bone


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CARTILAGE

FUNCTIONS:

- Bears mechanical stress

- Support soft tissues- Shock absorber & sliding area for joints

- For development & growth of long

bones


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CARTILAGE

Chondrocytes

(lacunae)

- synthesize &

secrete theextracellular

matrix

- active

Extracellular

matrix


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CARTILAGE EXTRACELLULAR MATRIX

1. Proteoglycans - sulfated GAG (chondroitin

sulfate & keratin sulfate) predominate w/

hyaluronic acid, forming central backbone of thecomplex

2. Collagen & elastic fibers electrostatic bonds b/w

collagen fibers & GAG side chains; binding of

water to (-) charged GAG chains3. Chondronectin promote the adherence of

chondrocytes to matrix collagen


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CARTILAGEStructurally, proteoglycans

resemble bottle brushes:

*stem protein core

*bristles GAG chains

Linking proteins:noncovalently bind coreprotein of PG to linearhyaluronic acid

Chondroitin sulfate side chains

of PG electrostaticallybind to the collagen fibrils,forming a cross-linkedmatrixSchematic diagram of a

cartilage matrix


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CARTILAGE

3 MAIN TYPES:(variations in matrix composition)

1. HYALINE most common;

matrix: type II collagen

2. ELASTIC collagen II + elastic fibers

3. FIBROCARTILAGE

- weight bearing regions of the body- matrix: dense network of coarse type I

collagen fibers


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HYALINE CARTILAGE

Found in:

1. Resp. tree up to bronchi

2. Sternal ends of ribs3. Most articular surfaces of joints

4. Epiphyseal plates of long bones

Precursor of bones in the developing

skeleton


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HYALINE CARTILAGE

ROUTINE HISTOLOGIC

PREPARATIONS collagen not

demonstrated by light

microscopy:1. Collagen fibrils have

submicroscopic dimensions

2. Similarity of the staining with

the matrix media3. Almost the same refractive

index of the collagen fibrils and

the ground substance


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HYALINE CARTILAGEPerichondrium- dense CT, covers all hyaline

cartilage

- growth & maintenance

- Collagen I & fibroblasts

- Layers:

1. Fibrous mostly collagenous

fibers & few fibroblasts; merges w/

subepithelial CT

2. Chondrogenic more cellular;

chondroblasts & chondrogenic cell

Inner Zone strongly basophilic


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HYALINE CARTILAGE

Inner Zone

(Chondrocytes)

- round

- lacunae

- isogenous grps

- considerable

shrinkage instandard

preparations


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HYALINE CARTILAGE

Interterritorial matrix

Territorial / capsular

matrix

- rich GAG, less collagen- Basophilic,

metachromatic

Inner to outer- Morphologic gradations

- Peripheral: young

chondrocytes


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HYALINE CARTILAGE

Mature chondrocyte- Small nuclei,

dispersed chromatin,

basophilic granular

cytoplasm

- Lipid droplets

- Cytoplasm rich in

glycogen

*Active in synthesis of

both groundsubstance and

fibrous elements


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CARTILAGE GROWTH

Stimulated by:

1. Growth hormone

2. Thyroxine3. testosterone

Retarded by: cortisone, hydrocortisone,estradiol

Somatotropin somatomedin C acts

directly on cartilage to promote growth


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CARTILAGE GROWTH

2 PROCESSES:

1. Interstitial growth results from mitotic

division of isolated chondrocytes to

form clusters

2. Appositional growth is the process by

which cartilage extends its borders;

occurs when chondroblasts of theperichondrium differentiate into

chondrocytes


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CARTILAGE GROWTH

Interstitial Growth

- Less important; young cartilage

- Epiphyseal plates of long bones- Cartilage model (endochondral bone

formation)

- W/in articular cartilage (no

perichondrium)- Becomes less pronounced as matrix

becomes increasingly rigid from cross-

linking of matrix components (cartilage

then grows in girth only by apposition)


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CARTILAGE NUTRITION

1. Through the water of solvation of the groundsubstance from capillaries in the perichondrium

2. Synovial fluid from joint cavities

*Chondrocytes exhibit low metabolic activity

limits thickness to which cartilage may develop

while maintaining viability of the innermost cells.


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CARTILAGE REGENERATION With difficulty and often incompletely

FRACTURES: chondroblasts from perichondrium

invade fractured area & generate new cartilage

In extensively damaged areas, perichondrium

generates a scar of dense CT instead of forming

new cartilage


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ELASTIC CARTILAGE

- similar to hyaline cartilage

except: for bundles of

branching elastic fibers in

the matrix

- Development & growth:interstitial & appositional

*Found in:

Ext. ear, EAC, epiglottis, larynx


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FIBROCARTILAGE

Intermediate features

Chondrocytes: singly or in clusters;

arranged in long rows

Alternating layers of amorphous matrix

& coarse type I collagen fibers No identifiable perichondrium

*Found in:IV disc, articular cart., symph. p.in assoc. w/ dense CT in joint

capsules, ligaments, tendons


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BONE

Composed of cells & collagenous extracellular matrix

(type I collagen) called OSTEOID, which becomes

mineralized by the deposition of calcium

hydroxyapatite

FUNCTIONS:1. Forms the strong & rigid endoskeleton

2. Protects the vital internal organs

3. Acts as reservoir ofCA, PO4 & other ions

4. Houses the hematopoietic bone marrow


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BONE

Special techniques for its preparation:

< GROUND SECTION

- does not preserve the

cells; permit detailed

study of the matrix,lacunae, canaliculi

DECALCIFIED BONE

TISSUE >


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BONE

BONE CELLS:

1. Osteoblasts Synthesize organic matrix

2. Osteocytes Inactive obsteoblasts trapped in bone

3. Osteoclasts Multinucleated giant phagocytic cellsimportant in resorption & remodeling of bone tissue

*Osteoblasts & Osteocytes are derived from a primitive

mesenchymal cell called OSTEOPROGENITOR

CELLS

*Osteoclasts are derived from macrophage-monocyte cell

line


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BONE


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OSTEOBLASTS

Synthesize osteoid & mediate its

mineralization

Lined up along bone surfaces

Active: cuboidal to columnar; basophiliccytoplasm

Cytoplasmic processes bring them in contact

with each other; lacunae & canaliculi appear

because matrix is formed around them

Once surrounded by newly synthesized matrix,OSTEOCYTE. This process, BONE

APPOSITION, is completed by subsequent

deposition of calcium salts into the newly

formed matrix


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OSTEOCLASTS Large, multinucleated, extensively

branched motile cell containing 5-50(or even more) nuclei

HOWSHIPS LACUNAE: depressionsresorbed from the bones surface

RUFFLED BORDER: aspect ofosteoclast in apposition to bone;secretes several organic acids whichdissolve the mineral component whilelysosomal proteolytic enzymes areemployed to destroy the organicmatrix

CLEAR ZONE: devoid of organelles,yet rich in actin filaments(microfilaments)

BASAL REGION: nuclei located


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OSTEOCLASTS Osteoclastic resorption contributes to

bone remodelling in response togrowth or changing mechanicalstresses upon the bone

The interactions b/n osteoclasts &osteoblasts are exquisitely regulated

during normal formation &remodelling of bone

Osteoclasts also participate in thelong-term maintenance of bloodcalcium homeostasis

PARATHYROID HORMONE:

CALCITONIN:

Stimulatesosteoclastic resorption & release

Ca ions from bone

Inhibits osteoclastic

activity


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TYPES OF BONE (GROSS)1. Compact (cortical) bone

dense areas w/o cavities

2. Cancellous bone(spongy/medullary) areas or

trabeculae w/ numerousinterconnecting cavities

*Spongy & compact bone are differentarrangements of the samecomponents; the trabeculae of

spongy bone consist of a numberof lamellae

*Sharing the same cavity with thespongy bone is the marrow


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TYPES OF BONE (Histologic)1. Primary/immature/woven bone - immature

form w/ randomly arranged

- formed when ostoblasts produce osteoid rapidly

(fetal bone devt & adult pathologic rapid new

bone formation)

- the rapidly formed woven bone is eventually

remodelled to form lamellar bone

- compared to secondary bone:

lower mineral content

higher proportion of osteocytes

2. Secondary/mature/lamellar bone virtually allbone in a healthy adult

- composed of regular bands of collagenarranged

in lamellae that are parallel to each other or

concentrically organized around a vascularcanal


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PERIOSTEUM & ENDOSTEUM

*layers of bone-forming cells & CT

Periosteum external surface of bone

- richly supplied w/ b.v.

- Sharpeys fibers: bundles of collagen fibersw/c bind periosteum to underlying bone

Layers:

1. Outer fibrous collagen fibers & fibroblasts

2. Inner, more cellular layer flattened

osteoprogenitor cells -> mitosis:osteoblasts

Endosteum internal surface

- thinner, single layer of flattened

osteoprogenitor cells, very small amount of CTEndosteum


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HAVERSIAN SYSTEM (Osteon)The whole complex of concentric bony

layers or lamellae disposed around a central

channel containing blood vessels,

lymphatics, nerves and loose CT

Canals of Havers or Haversian canals

Volkmanns canals

Each Haversian system begins as a broad

channel. With deposition of successive

lamellae: dec. diameter of canal,

osteoblasts trapped as osteocytes in

lacunae. Osteocytes are thus arranged in

concentric rings w/in lamellae

Canaliculicontain fine cytoplasmic

extensions

Interstitial systems

Inner & outer circumferential lamellae


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HAVERSIAN SYSTEM


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BONE FORMATION

1. Intramembranous ossification By direct mineralization of matrix secreted by

osteocytes

2. Endochondral ossification By deposition of bone matrix on a pre-existing

cartilage matrix

In both processes, the bone tissue that appears

first is primary or immature


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Intramembranous

ossification Source of most flat bones Growth of short bones & thickening of

long bones

Occurs w/in membranes of

condensed primitive mesenchymaltissue. Primary ossification centeris

the starting point of ossification.

Anastomosing trabeculae (T) of

forming bone darkly stained; lighter

embryonic CT (highly vascular) Bony trabeculae forming primitive

osteons (Os) surrounding HC

Osteocytes arranged haphazardly

Every trabecula covered w/ osteoblasts


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Intramembranous

ossificationMesenchymal cells osteoblast synthesize& secrete osteoid at multiple ossificationcenters mineralization of osteoid osteoblasts trapped in lacunae, cytoplasmicextensions in canalicuki. SPICULES formed

& fuse (spongy bone) remaining CTpenetrated by growing b.v. (Remainingmesenchyme differentiate into bonemarrow)

osteoprogenitor cells undergo mitotic division,more osteoblasts lay down more bone

several ossification centers grow radially & finallyfuse together, replacing the original CT

*The portion of the CT layer that does notundergo ossification gives rise to the endosteum& periosteum of the intramembranous bone


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Endochondral

ossification- A small model of solid hyaline cartilage of longbone is 1st formed

- Perichondrium (w/ osteogenic potential): laysdown bone collar

- Chondrocytes hypertrophy, resorbingsurrounding cartilage

- Cartilage matrix calcified, chondrocytes die,leaving large interconnecting spaces

- Osteogenic bud: invades spaces left

- Primitive mesenchymal cells differentiate intoosteoblasts; form layer on surface of calcifiedcartilage matrix synthesize bone matrix

- Calcified cart. matrix resorbed by giant cells

- BM stem cells from circulation brought byosteogenic bud into forming bone

- Primary ossification center: accompanied byexpansion of periosteal bone collar


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Endochondral

ossification- At the same time, osteoclasts are active;resorption of the bone occurs at the center to

leave a large medullary space filled w/ bone

marrow

- Secondary ossification center in epiphysis:

growth is radial instead of longitudinal

*Articular cartilage: does not contribute to bone

formation

*Growth or Epiphyseal plate/cartilage: On

reaching maturity, hormonal changes inhibitfurther cartilage proliferation and the

epiphyseal plates are replaced by bone,

causing fusion of diaphysis & epiphysis


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EPIPHYSIS


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Epiphyseal CartilageFunctional & Morphologic Zones (from

epiphyseal side)

1. RESTING ZONE typical hyaline cartilagew/o morphologic changes in the cell

2. PROLIFERATIVE ZONE successivemitotic divisions to form columns ofstacked cells

3. HYPERTROPHIC CARTILAGE ZONE chondrocytes enlarge & becomevacuolated. Matrix is resorbed

4. CALCIFIED CARTILAGE ZONE Matrix iscalcified

5. OSSIFICATION ZONE Endochondralbone tissue appears; osteogenic cells &capillaries invade calcified matrix;osteoblasts deposit bone matrix overcalcified cartilage

bone & cart lecture

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