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Alveolar bone in health and disease

Presented by:Snehi Kumar

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CONTENTS :DEFINITONBRANCHIAL ARCHESDEVELOPMENT OF MANDIBLEDEVELOPMENT OF MAXILLAINTRODUCTIONDEVELPOMENT OF ALVEOLAR PROCESSSTRUCTURE OF ALVEOLAR PROCESS

Alveolar bone properSupporting boneInterdental septumSocket wallBone marrowPeriosteum and Endosteum

COMPOSITIONBONE CELLS

-Osteoblast -Osteocyte -Osteoclast

MECHANISM OF BONE FORMATIONMECHANISM OF BONE RESORPTIONCLINICAL CONSIDERATION

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ALVEOLAR PROCESSThe alveolar process is that portion of the maxilla and the mandible that forms and supports the tooth sockets (alveoli) . However there is no boundary between the alveolar process and the basal bone of the jaws.

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The formation as well as continued preservation of the alveolar bone is dependent on continued presence of teeth.

Its morphologic characteristics being related to :Size of teethShape of teethEvents occurring during tooth eruption Inclination of erupted teeth

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Normal bone contour conforms to the prominence of the roots.

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DEVELOPMENT

Stomodeum formation

Laterally bounded by a pair of branchial or pharyngeal arches

Sheet of mesoderm sandwiched between ectoderm and endoderm.

Ectodermal pouch and cleft divide into 6 thicknings

The first branchial arch forms other processes - the maxillary process and mandibular process.

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Branchial arches

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Development of the mandibleIntramembranous development

Develops indirectly from the Meckles cartilage which has a close relationship with the Trigeminal nerve(mandibular division).

Meckles cartilage extends as a solid hyaline cartilage on both sides.

Mandibular division of trigeminal nerve divides into

Inferior alveolar

Lingual branch

incisive mental

The intramembranous ossification starts at the 7th week both : Anteriorly Posteriorly

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Anteriorly Posteriorl

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Lateral plat

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Medial plat

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Lateral plat

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Medial plat

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Trough is formed Gutter is formed

Soon converted into a canal as bone is formed.

By rapid spread of ossification posteriorly into mesenchyme of first arch the RAMUS is formed and the point of divergence being marked by lingula.By the end of the 10th week – the rudimentary mandible is formed.

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Spread of mandibular ossification away from Meckel’s cartilage at the lingula.

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The further growth of the mandible is governed by the SECONDARY CARTILAGE :

CONDYLAR PROCESSCORONOID PROCESSSYMPHYSEAL PROCESS

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DEVELOPMENT OF THE MAXILLAThe center of ossification is the mesenchyme of the maxillary process of the first

arch,from where the maxilla starts to develop.

The center of ossification is associated closely with the cartilage of nasal capsule , no arch catilage exists in the process.

The centre of ossification develops between the branches of antero superior dental nerve (branch of the inferior orbital nerve )

Ossification spreads

Posteriorly Superiorly

Inferiorly

Lateral alveolar plate Medial alveolar plate

Secondary cartilage: zygomatic or malar process contributing to zygomatic process..

(And anteriorly upto the incisor region)

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INTRODUCTIONTogether with the root cementum and periodontal membrane the alveolar bone forms the “attatchment apparatus”

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The alveolar process develops in conjugation with the development and eruption of the teeth .

The alveolar process consist of the bone which is formed by *cells from dental follicle(alveolar bone proper)

At 8 weeks in utero alveolar bone forms a horse shoe shaped groove which contains the tooth germs.

Gradually bone develops between the roots and soon a crypt is formed.

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At the end of second month of fetal life the maxilla as well as the mandible from a groove known as the dental groove.

The alveolar process at the time of birth are known as gum pads.

They are pink ,firm and are covered by a dense layer of fibrous periosteum.

Initially embryonic type of bone is formed which is gradually replaced by mature lamellar bone.

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Formation of alveolar boneBone development around erupting tooth

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Cortical bone(external plate)

Compact bone(Alveolar bone

proper)

Cancellous bone/Trabecula

r boneBasal bone

The alveolar process consists of the following :

Haversian bone

Compacted bone lamellae

Seen as lamina dura in radiographs.Has series of openings for neurovascular bundles hence known as cribriform plate.Acts as supporting

alveolar bone and is enclosed by a compact border between the interdental septum.

Portion of jaw located apically, unrelated to teeth.

Bone marrow Periosteum Endosteum

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Parts of alveolar bone

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The alveolar bone proper consists of thin lamella of bone1. That surrounds the root of the tooth2. Gives attachment to principle fibers of periodontal ligamentThe compact bone which lines the tooth socket and in radiographs it appears as “Lamina dura.”

ALVEOLAR BONE PROPER

The layer of bone into which the principal fibre are inserted is sometimes called “Bundle bone”.Alveolar bone or the bundle bone has a varying width.

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Alveolar process with periodontal ligament and cementum is responsible for the attachment between the tooth and the skeleton.

Alveolar bone proper as a result of altered functional demands undergo adaptive changes.

The alveolar bone which forms the inner wall of the socket is perforated by many openings that carry branches of the inter alveolar nerves and blood vessels into the periodontal ligament and is therefore called the “cribriform plate”.

Allignment of bony

trabeculae

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Histopathological features :

The alveolar bone includes circumferential lamella and contains sharpey’s fibre which extend into periodontal ligament .

Lamellar bone at a site contains osteons each of which harbours blood vessels located in haversian canals.

The space between the different osteons is filled with so called interstitial lamellae .

Histology of alveolar bone proper

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Compact bone is perforated by numerous Volkman canals through which

Blood vessels

Nerves

Lymphatics

pass from alveolar bone to the periodontal ligament.

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In maxilla the outer cortical plate is perforated by many small openings through which blood and lymph vessels pass.

In lower jaw the cortical bone of alveolar process is dense in the region of anterior teeth.The supporting bone is usually very thin,no spongy bone is found here and the cortical plate is fused with the alveolar bone proper.

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Cortical Plates

The cortical plates form the outer and inner plates of the alveolar process.

The cortical plates are continuous with the compact layer of maxillary and mandibular body.

They are generally much thinner in the maxilla than in the mandible.

They are thickest in the premolar and molar regions of the jaw,Specially on the buccal side.

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Interdental Septum

The interdental septum consist of cancellous supporting bone enclosed within a compact border.

The interdental and interadicular septa contains the perforating canals of Zuckerkandl and Hirschfeld (nutrient canal)which house the:•interdental and interadicular arteries•veins •lymph vessels•nerves

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Socket Wall

The socket wall consist of dense lamellated bone,some of which is arranged in haversian system and bundle bone.

It is characterized by thin lamellae arranged in layers parallel to the root with intervening oppositional line.

Bundle bone is localized within alveolar bone proper.Some sharpey’s fibres are completely calcified but most contain an uncalcified outer layer .

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Bone Marrow

•In the embryo ,the cavities of all bones are occupied by a red hematopoietic marrow

•The red marrow gradually undergo a physiologic change to a fatty or yellow inactive type of marrow

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Periosteum

It is a tough sheath of dense irregular connective tissue.

It surrounds the bone surface whenever it is not covered by articular cartilage.

The periosteum contains bone forming cells .Functions :i. Allow the bone to grow in

thicknessii. Assists in fracture repairiii.Helps to nourish bone

tissueEndosteum

It is a membrane that contains bone forming cells and lines the medullary cavity.

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COMPOSITION of Alveolar Bone :

65% Inorganic

35% Organic COLLAGEN 88-89%

NON-COLLAGEN 11-12%

GLYCOPROTEINS 6.5-10%

PROTEOGLYCANS 0.8%

SIALOPROTEINS 0.35%

LIPIDS 0.4%

The organic material is primarily type I collagen which lies in the ground substance.Glycoprotein are proteins with small amount of monossacharides,disaccharides, and polysaccharides or oligo -saccharides and proteoglycans are sulfated and non-sulfated glycosoaminoglycan with a small amount of protein.

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The inorganic material almost exclusively consist of calcium and inorganic orthophosphate in the form of hydroxyapatite crystals .

The crystals are deposited in between the molecules of collagen as well as in the non-collagen,organic material that makes up the ground substance of bone.

The exact mechanism by which hydroxyapatite crystals are deposited in the bone matrix produced by osteoblasts is still unknown ,however certain enzymes like Alkaline Phosphatase , ATP ase and Pyrophosphatase have been shown to participate in this process .

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STRUCTURAL CHANGES

The internal structure of bone is adapted to mechanical stress.

Its changes continues during growth and alteration of functional stress.

In jaws ,structural changes are correlated to the

Growth

Eruption

Movement

Wear

Loss of teethAll these process are made possible only by a coordination of destructive and formative activities ,these are brought about by : OSTEOBLAST OSTEOCLAST

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Cells :OSTEOBLASTS

Osteoblasts are mononuclear -plump,cuboidal cells or slightly flattened cells ,that are primarily responsible for bone production.

Although they are post mitotic cells both preosteoblast and osteoblast can undergo mitosis during prenatal development and occasionally during post natal growth.

Both have high level of alkaline phosphatase on the outer surface of plasma membrane.

This enzyme is used experimentally as a cytochemicalmarker to distinguish preosteoblast from fibroblast.

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Osteoblasts exhibit abundant and well developed protein synthetic organelles.

-At light microscope the golgi complex characteristically appear as a clear , paranuclear area that can be defined easily following cytochemical reaction . For golgi-resistant enzyme the collagen type I molecule is formed and assembled, as in fibroblasts and odontoblasts within the spherical and cylindrical distention of the golgi complex.

Apart from that it consists of rough endoplasmic reticulum,mitochondria, nucleoli,and many secretory vesicles and vacuoles.

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Their function is synthesis of:

Collagen Non-collagen

Mainly osteoid

I. Type –I and TYPE –V collagenII.ProteoglycansIII.Several non collagen protein

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The typical elongated ,electron dense, collagen –containing secretory granules release their content primarily along the surface of the cell apposed to forming bone,these molecules assemble extracellularly as fibrils to form the osteoid layer.Irrespective of this aspect non-collagenous protein also are released mainly along the surface of osteoblast apposed to osteoid and diffuse from the surface towards the mineralization front where they participate in regulating mineral deposition.Near the mineralization front ,mineralization foci can be seen within osteoid and certain non collagenous proteins, such as bone sialoproteins and osteopontin accumulating within them .Osteoblast secrete several bone morphogenic protein (BMP) superfamily including :-BMP-2,BMP-7 andTransforming growth factor(TGF)in addition to ;Insulin –like growth factor (IGF-I)Platelet derived growth factor(PDGF)Fibroblastic growth factor(FGF)

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Although the timings of secretion and complex interaction of these growth factors remain to be clarified;the combination of IGF-I,TGF-β and PDGF, and fibroblastic growth factor (FGF) exists .

For instance these combinations ,may be used to speed healing and bone growth after periodontal surgery or to prevent periodontal disease by the early treatment of periodontal pocket.

Similarly ,they may be used to enhance osseous integration after placement of dental implants.

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OSTEOCYTES

As osteoblast form ,some become entrapped within the matrix they secrete ,whether mineralized or non-mineralised ,these cells are then called osteocytes.The number of osteoblast that become osteocytes are present per unit volume .

As a general rule ,embryonic bone and repair bone have more osteocytes than does a lamellar bone .

After bone formation ,osteocytes become reduced in size , space in the matrix occupied by osteocytes is called “osteocytic lacunae”.Narrow extentions of these lacunae form enclosed channels or cannaculi that house radiating osteocytic process through these channels.

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Compared with all other bone cells and their precursors ,the multinucleated osteoclast is a much larger cell,because of their size.Osteoclast can be identified easily under the light microscope, they often are seen in cluster.

OSTEOCLASTS

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Typically osteoclast are found against the bone surface ,occupying hollowed –out depression calledHOWSHIPS LACUNAE,that they have created .

Scanning electron microscopy of bone resorbing surface show

The howships lacunae reflecting the activity and the mobility of osteoclast during active resorption under the electron microscope,multinucleated osteoclast exibit a unique set of morphological characteristics.

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The cell organelles consist of many nuclei each of which is surrounded by multiple golgi complexes ,mitochondria , numerous vesicular structures situated between the golgi complex and resorption surface .

For years osteoclast have been known to be rich in lysosomal structures.Enzymes are synthesized in the rough endoplasmic reticulum later transported to the golgi complexes and then move to the ruffled border.

An electron dense,interfacial matrix layer (lamina limitans)often is observed between the sealing zone and calcified tissue surface .

Several mechanism bind the osteoclast to surface,among these the concentration of osteopontin on the bone surface (lamina limitans) may facilitate osteoclast adhesion and formation of the sealing zone .

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Adjacent to the tissue surface ,the multinucleated osteoclast cell membrane is thrown into a myriad of deep folds that form a “ruffled border”.

At the periphery of this border ,the plasma membrane is opposed closely to the bone surface and the adjacent cytoplasm devoid of cell organelles and is enriched in

Actin VinculinTalinFibrillar contractile protein

This clear or sealing zone not only attaches the cell to the mineralized surface but also isolates a microenvironment between them and the bone surface .

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ACTIN is a globular, roughly 42-kDAprotein found in all  eukaryotic cells where it may be present at concentrations of over 100 µm.Actin is the mononumeric subunit of two types of filaments in cells: microfibril, a major component of the cytoskeleton, and thin filaments, part of the contractile apparatus in muscle cells. Thus, actin participates in many important cellular processes including muscle contraction, cell motility, cell division and cytokinesis, vesicle and organelle movement, cell signaling, and the establishment and maintenance of cell junctions and cell shape. 

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VINCULIN is a cytoskeletal protein associated with cell-cell and cell-matrix junctions, where it is thought to function as one of several interacting proteins involved in anchoring F-actin to the membrane. Binding alternately to talin or α-actinin, vinculin's shape and, as a consequence, its binding properties are changed. The vinculin gene occurs as a single copy and what appears to be no close relative to take over functions in its absence.

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TALIN is a high-molecular-weight cytoskeletal protein concentrated at regions of cell-substratum contact and, in lymphocytes, at cell-cell contacts.Talin is a ubiquitous cytosolic protein that is found in high concentrations in focal adhesions. It is capable of linking integrins to the actin cytoskeleton either directly or indirectly by interacting with vinculin and alpha-actinin.

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Mechanism of bone formationI. Membranous bone formation

II. Endochondral bone formation

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As the osteoblasts secrete the organic matrix of bone ,it is at first devoid of mineral salts and at this stage it stains pink in routine hematoxyline and eosin and is called osteoid tissue ,as this material is produced, some of the osteoblasts become embedded in it and form the osteocytes.

Near the mineralization front ,mineralization foci can be seen within osteoid and certain non collagenous proteins,such as bone sialoproteins and osteopontin accumulating within them .

In addition to structured matrix protein ,osteoblasts secrete a variety of cytokines that help regulate cell metabolism

A key factor in the rate of bone cell development is the elaboration of number of growth factors by osteoblast ,their precursor or both.

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Bone resorptionBone resorption at any site is a chemotactic phenomenon i.e it is initiated by the release of some soluble factors that attracts monocytes to the target site.

During bone resorption three process occur :-

Decalcification Degradation of

matrix

Transport of soluble

products to the extracellular fluid or the

blood vascular system

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Since calcified matrix is resistant to protease of all kinds bone must be decalcified ,this is achieved at the ruffled border of osteoclast by the secretion of organic acid (citric and lactic acid),which chelate bone and by H+ which increases the solubility of hydroxy apatite.

DECALCIFICATION

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After this decalcification process pieces of matrix are released by the activity of cathepsin B1 and collagenase enzyme.

Collagenase is secreted as a proenzyme that is activated by specific neutral protease .

Collagenolytic activity takes place outside the osteoclast and occurs at a specific site on tropocollagen (collagen)molecule .The site is one-third of the distance from the carboxyl end of the molecule .

The broken fragments of collagen are further decalcified and collagenolysis occur outside the osteoclast ,only calcium phosphate can be identified within these cells.

DEGRADATION OF MATRIX

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TRANSPORTATION OF SOLUBLE PRODUCT

After the degradation of the matrix the breakdown products of bone must be transported to the extracellular fluid and to the blood vascular system.

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FACTORS AFFECTING BONE FORMATIONThe hormones most important in bone metabolism are : a) Parathyroid hormone (PTH)

b) 1,25,dihydroxyvitamin D

c) Calcitonin

d) Estrogen

e) Glucocorticoids

The action of parathyroid hormone and vitamins are biphasic enhancing bone resorption at high concentration ,but supporting bone formation at lower concentration .

Calcitonin +estrogen inhibit resorption where as the glucocorticoids inhibit formation .

The hormones effecting bone most likely work primarily through altering the secretion of the cytokines .The osteoblast and lining cells also have anabolic function and participate in matrix degradation through the production of interlukin -6.

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Bone remodellingBone remodelling takes place by mainly 3 processes:

Osteogenesis OsteoinductionOsteoconduction

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CLINICAL CONSIDERATIONS

ORTHODONTIC ASPECT

Bone although the hardest substance is biologically a highly plastic tissue.

Bone is resorbed on the side of pressure and opposed on the side of tension.

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Effect of pressure and tension on bone .

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Bone resorption is almost universal.

Occur in episodic spurts both in horizontal and vertical types.

Endotoxins released by gram negative bacteria of the plaque leads to an increase in cAMP which increase the osteoclastic activities.

Further more a peptide called osteoclast activating factor has been demonstrated in the lymphocytes near the periodontal pocket .

This substance is capable of increasing cAMP and osteoclastic activity and reducing osteoblastic activity at the target site.

PERIODONTAL ASPECT

The greater number of cells and the reduced volume of calcified intercellular substance render this immature bone more radiolucent than mature bone.

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References

ORBANS ORAL HISTOLOGY & EMBROYOLOGY-11EDITIONAVERY ORAL DEVELOPMENT & HISTOLOGY-3EDITIONCARRANZA-9&10 EDITIONLINDHE-4&5 EDITIONJOURNAL 2000 PERIODONTOLOGYA.R.TEN CATES book OF ORAL HISTOLOGYS.L.BHALAJHI ,TEXTBOOK OF ORTHODONTICSTHEORY AND PRACTICE OF HISTOLOGICAL TECHNIQUES – J.D.BANCROFT & M.GAMBLEK.B.BERKOVITZ-ORAL ANATOMY & EMBROYOLOGY

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