Alveolar Bone -Vandy

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Introduction

Classification

Composition

Histology Bone formation

Bone turnover

Bone remodeling


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Bones are calcified connective tissue forming the

major portion of the skeleton of most vertebrates

more calcium than any other organ

consists of a dense organic matrix and an

inorganic, mineral component


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Functions of the bone in brief;

toughness and elasticity

shape and support

site of attachment for tendons and

muscles

Protects vital organs of the body

Serve as a storage site for minerals provides medium for development and

storage of blood cells

marrow


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Bones may be classified according;

Shape.

Mode of development.

Histologic appearance.


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Classification based on shape:

Long bones.Flat bones.

Irregular bones.


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They are long and slender.

Longer than wider.


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Cube shaped of nearly equal length and width.

Consists of spongy bone covered by thin layer ofcompact bone.

Examples;

bones of wrist and ankle.


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Thin, flat ,curved with no marrow cavity.

spongy bone is present between upper and lower layer

of compact bone.

Examples; sternum,ribs,clavicle


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Have complex shapes, notched or with ridges.

Made of spongy bone covered with layer of compactbone.

Examples;

vertebrae,mandible, sphenoid,pelvic bones.


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Endochondral bones

Bones of trunk and extremities

Intramembranous bones Cranial and facial flat bones of the skull, mandible and clavicles.


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Histologically;

Two types;

Mature bone

Immature bone


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Mature bone

Two types

compact (cortical) bone

cancellous(spongy) bone


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Woven/immature bone.

First formed bone with irregularly oriented collagen fibers of varying

diameter.

Present in

alveolar bone

healing of fractures.


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67% 33%

inorganic organic

hydroxyapatite 28% 5%

collagenOsteocalcin

Sialoprotein

Phophoprotein

Osteonectin

Bone specific protein


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The mineral component; hydroxyapatite

thin plates or leaf like structures

packed closely with long axis nearly parallel to collagenfibril axis

The narrow gaps between the crystals contain water

and organic macromolecules. Ions present;

Carbonate,calcium phosphate,hydroxyl.


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5% of the total organic content of bone matrix.

Endogenous proteins produced by the bone cells.

Albumin is derived from the blood and become

incorporated into the bone matrix duringosteosynthesis.


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Osteocalcin; noncollagenous protein 15% of the noncollagenous bone protein. Also known as bone Gla protein

Aminoacid gamma carboxy glutamic acid.

Regulation by vitamin D and parathyroid hormone.


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Osteopontin and bone sialoprotein ;

heavily glycosylated and phosphorylated

high levels of acidic aminoacids

Aspartate Glutamic acid

Potent inhibitor of

hydroxy apatite

crystal growth.

Plays role in the

intiation of mineral

crystal formation.Transcription is

upregulated by

vitamin D3

suppressed by

vitamin D3

osteopontin sialoprotein


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Osteonectin

25% of noncollagenous proteins

bound to hydroxyapatite crystal

regulation of cell adhesion

proliferation and modulation of cytokine activity


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Proteoglycans

A large chondroitin sulphate proteoglycans

Non mineralized bone matrix

Small proteoglycans, biglycan and decorin(


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Lysyl oxidase(collagen cross linking) and tyrosine rich

acidic matrix protein(TRAMP) Demineralised bone and bone matrix

TRAMP( dermatopontin) binds decorin and TGF-beta

Regulate the cellular response to TGF beta

Others


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Dense outer sheet ofcompact bone and a central

medullary cavity

Cavity is filled with red or yellow bone marrow

interrupted, particularly at the ends of long bones, bya network of bone trabeculae ie.trabecular, cancellous,

or spongy bone


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Circumferential lamellae;At the periosteal and endosteal surfaces, the lamellae inparallel layers surrounding the bony surface

Concentric lamellae;Deep to the circumferential lamellae, the lamellae are arranged

in concentric layers around a central vascular canal


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Interstitial lamellae: are interspersed between

adjacent concentric lamellae and fill the spaces

between them


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Red

marrow

young

bone

yellowmarrow

oldbone


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osteoblasts

osteocytes

osteoclast


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mononucleated cells

synthesis and secretion of macromolecular

organic constituents of bone matrix

osteoprogenitor cells of mesenchymal origin


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undifferentiated pluripotent stromal stem cells

differentiate into inducible osteoprogenitor cells(IOPCS)

IOPCs

Determined osteoprogenitor cells(DOPCS)

osteoblast

Osteoblasts


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Formation of new bone via synthesis of various proteins

and polysaccharides

Regulation of bone remoldeling and mineral metabolism

In the mineralization of osteoid

Osteoblasts


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Bone resorbing factors

parathyroid hormone,

vitamin D3,interleukin1

tumor necrosis factor

Receptor for these bone resorbing agents

Recognize the resorptive signal and transmit it to the

osteoclast

Osteoblasts


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The overall integrity of bone is controlled by;

Hormones

Protein secreted by hematopoietic bone marrow

cells

Bone cells

Osteoblasts


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In response of hypocalcemia;

hormone activates the mechanism for the release ofcalcium from the bone

PTH does so;

indirect effect mediated by PTH receptors on bonestromal cells including osteoblast

Osteoblasts


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Stimulates bone resorption

Essential for normal bone growth and mineralisation

Promotes calcium absorption from the intestine

stimulates synthesis of osteocalcin and osteopontin by

osteoblast

suppresses collagen production

Osteoblasts


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Parathormone and vitamin D3 enhance;

bone resorption at high concentration(pharmacological) bone formation at low(physiological)

Osteoblasts


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Required for attaining normal bone mass;

Mediated by the local production of IGF-1

Binds to membrane growth hormone receptors on

activated osteoblast

Osteoblasts


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It targets osteoblast directly

Stimulates bone matrix formation

Mineralisation Indirectly affects bone formation through

stimulation of IGF-1

Osteoblasts

Osteoblasts


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Formation of new bone including;

Migration

Aggregation

Proliferation of mesenchymal type cells

Differentiation into osteogenic cells

Osteoblasts


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Osteoblasts after, completing their function got

entrapped in bone matrix and become osteocytes or

remain on the surface as the lining cells


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Osteoblasts form the bone matrix, they got entrapped

within the matrix they secrete and are called as

osteocytes

The number of osteoblasts that become osteocytes,

depend on the rapidity of bone formation


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Osteocytic lacunae;

within the bone matrix, the osteocyte reduce in

size creating a space around it

ovoid or flattened

Osteocytes


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Narrow extensions of these lacunae forms channels

called canaliculi

Osteocytic processes are present within these

canaliculi;

contain bundles of microfilaments and smooth

endoplasmic reticulum.

Osteocytes


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At the distal end, these processes contact

the processes of adjacent

cells(osteocytes)

contact with osteoblasts and bone liningcells at the surface


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The word osteoblast is derived from the Greek words

for bone and broken

Removes bone tissue by removing the mineralized

matrix of bone


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Osteoclasts lie in resorption bays called Howships

lacunae.

large cell approximately 40-100m in diameter with 15to 20 closely packed nuclei.

multinucleated osteoclast resorb more bone than with

few nuclei.

Osteoclasts


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The cytoplasm also microtubles which transport vesicles

between golgi stacks and ruffled membrane

Cathepsin containing vesicles and vacuoles are present

close to the ruffled border indicating resorptive activity

of these cells

Osteoclasts


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Haemopoietic cells of monocyte macrophage

lineage

Proliferate and differentiate into osteoclasts

through a mechanism involving cell-cell

interaction with osteoclast stromal cells

Osteoclasts


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The formation requires the presence of RANK ligand

and M-CSF

Neighboring stromal cells and osteoclast precursor

RANK Ligand and M-CSF

Osteoclasts


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RANKL;

Differentiation into mature osteoclast

osteoclast activity

RANK is expressed by osteoclast precursors, a

membrane bound TNF receptor that recognizes RANKLthrough direct cell to cell interaction with osteoblast

or stromal cells

Osteoclasts

Osteoclasts


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The soluble TNF receptor family member OPG is a

natural RANKL antagonist ;

inhibit osteoclast formation and bone resorption.

Estrogen suppresses the ;

production of bone resorbing cytokines including IL-1

and IL-6.


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Calcitonin inhibits;

proliferation differentiation of osteoclast precursors

reduces the dimension of ruffled border and

dissociation into monocytic cells

Osteoclasts


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Three mechanisms

Endochondralbone growth

Intramembranousbone growth

Sutural bonegrowth


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Take place when the cartilage is replaced by bone.

Site;

At the ends of long bones

Vertebrae

Ribs

Head of the mandible

Base of the skull


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Mesenchymal cells

condenses and differentiate into chonbroblast

cartilage matrix(hyaline cartilage model)


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Hyaline cartilage model is surrounded by

perichondrium

Perichondrium;two layers Inner chondrogenic layer

outer fibrous layer


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The growth of cartilage model;

Intersititial growth

Appositional growth

Intersititial growth

Increase in the length

repeated division of chondrocytes

production of additional matrix by the daughter

cells.


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Appositional growth

Widening of the model

addition of matrix to its periphery by new

chondroblasts


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Zone of proliferation

Zone of hypertrophyand maturation

Zone of provisionaland mineralization

As the differentiation of cells

move towards metaphysis.

Cells organize into longitudinal sections


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Zones of proliferation;

The cells are small and flat

Constitute a source of new cells


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The chondrocytes hypertrophy

Secrete Type II collagen

As hypertrophy proceeds, proteoglycans are secreted Partial breakdown of proteoglycans, creating a matrix

environment receptive for mineral deposition

Zone of hyper t rophy and

maturat ion


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Formation of matrix vesicles

These membrane bound vesicles bud off from the cell

and form independent units in the longitudinal septa of

the cartilage

Zone of prov is ional m ineral isat ion


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In this bone develops directly within the soft connective

tissue rather than on a cartilaginous model


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It involves following steps;

Formation of bone matrix within the fibrous

membrane

The mesenchymal cells proliferate and condense

As vascularitry increases at the sites of condensed

mesenchyme, osteoblasts differentiate and begin to

produce bone matrix de nova


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At the multiple sites within each bone of the cranial

vault

MaxillaBody of the mandible

Midshaft of long bones


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The bone spicules gradually lengthen into longer

structures called as trabeculae

The trabeculae extend in a radial pattern and these

enclose blood vessels

Woven bone;early membrane bone

Periosteum ;external to woven bone,

condensation of vascular mesenchyme.


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In richly vascular areas, these osteogenic cells give

rise to osteoblasts that form the bone matrix

In areas, with no capillary blood supply, they form

the chondroblasts which lay down cartilage


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Osteoblasts and osteogenic cells

Proliferate in a highly vascularised environment

osteoblasts

Deposit new layers of bone matrix on preexisting bone

surface.


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Osteogenic cells are always in superficial position

repeating the process again and again

This is appositional growth which result in build of

bone tissue one layer at a time


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The number of narrow canals are formed linedby osteogenic cells

These canals enclose blood vessels(in soft tissue

spaces of cancellous network)

The consecutive lamellae of bone become added

to the bony walls of spaces in cancellous bone

which is called osteon or haversian system


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Permit skull and face to accommodate

growing organs such as eyes and brain

Same osteogenic potential as periosteum Skull bones forced apart by the growing

brain-bone forms at the sutural margins, with

waves of new bone cells differentiating from

the cambium


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Young immature bone is relatively thin, with few osteons.

Its periosteal surface is undulating and forms bone rapidly

Its endosteal surface is primarily for resorption.


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The immature bone grows.

Its periosteal surface is not as undulating and

produce large number of secondary osteons.

The primary osteons are resorbed.


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The bone nearly matures.it is thicker still, its periostealsurface is less undulating and teritary osteons replace

secondary osteons.Fragments of both primary

and secondary osteons forms interstitial lamellae


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It is the replacement of old bone by new

bone

Cutting cone;

The leading edge of resorption

it is characterized in cross section by scalloped

array of Howships lacunae,each housing anosteoclast


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Remodeling is the major pathway of bony changes in

shape, resistance to forces, repair of wounds, and

calcium and phosphate homeostasis in the body.


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Osteoclasts have resorbed organic matrix along

with hydroxyapatite

The breakdown of collagen from the organicmatrix releases various osteogenic substrates -

stimulates the differentiation of osteoblasts,

which - deposit bone


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Orbans: Oral Histology and Embryology 2000, 12th

edition.

Ten Cates Oral Histology, Development, structure and

function- 2005, 5th edition.

Alveolar Bone -Vandy

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