BONE FORMATION AND RESORPTION

PRESENTED BY,

Dr. Dandu Sivasai Prasad Reddy

1st yr PG

Mamata dental college

CONTENTS:•Introduction •Classification of bones Shape Development: Intramembranous ossification Immature and mature bone Intracarilaginous bone formation Microscopic structure•Composition of bone•Bone cells Osteoblasts Bone lining cells Osteocytes Osteoclast

•Bone formation and factors affecting bone formation•Bone resorption Differences between resorbed and unresorbed surfaces Role of TRAP in bone resorption Factors affecting bone resorption•Bone remodelling Sequence of events Mediators Markers of bone turn over•Conclusion • References

INTRODUCTION

CLASSIFICATION OF BONES:

Shape development histology

Long flat irregular endochondral Intramembranous sutural mature immature

compact cancellous

short

CLASSIFICATION…

Based on shape: Long bones Short bones Flat bones Irregular bones

BASED ON SHAPE

Limbs CarpalsTarsals

Vertebrae Skull bones, RibsSternum, Scapula

CLASSIFICATION…

BASED ON DEVELOPMENT:ENDOCHONDRAL BONES:

Intra cartilaginous or endochondral bone formation or indirect bone formation:

CLASSIFICATION…

INTRAMEMBRANOUS BONES:Intramembranous ossification/direct

bone formation

SUTURAL BONES:Growing face and skull.

CLASSIFICATION…

BASED ON MICROSCOPIC STRUCTURE:

COMPOSITION OF BONE:

Bone

33% organic

28% collagen

5% non collagenous proteins

67% inorganic

Hydroxyapatite

Composition of Bone…

Inorganic component:

Hydroxyapatite crystals with carbonate content

Organic component:- Osteoid

Type I collagen (95%) type V collagen (<5%)

Non collagenous proteins Osteocalcin, Osteopontin, Bone sialoprotein, Osteonectin.(SPARC)- Cell adhesion ,proliferation,

modulation of cytokine activity.

Osteoblasts :

Derived from osteoprogenitor cellsPeriosteum serves as important reservoir .

Morphology :

basophilic

cuboidal or slightly elongated cells

contain prominent bundles of actin, myosin

BONE CELLS:

Formation :

Pleuripotent stem cells

IOPC’S

DOPC’S

BMP’s ,GF’s

Osteoblasts

FUNCTIONS

New bone formation

Controls bone mineralization at 3 levels-

i. In its initial phase, by production of matrix vesicle.

ii. At a later stage, by controlling the ongoing process of

mineralization.

iii. By regulating the number of ions available.

Regulation of bone remodeling and mineral metabolism.

FUNCTIONS

Osteoblasts secrete type I collagen, small amount of

type V collagen, osteonectin, osteopontin, RANKL,

osteoprotegerin, Proteoglycans, latent proteases and

growth factors including bone morphogenic proteins.

Osteoblasts exhibit high levels of alkaline phosphatase -

cytochemical marker.

Vitamin D3:

Stimulates bone resorption.

essential for normal bone growth and mineralization

Stimulates osteopontin and osteocalcin – suppresses collagen

production

Growth hormone:

required for attaining normal bone mass - mediated by local

production of IGF-1.

Insulin:

stimulates bone matrix formation and mineralization

Bone morphogenic proteins :

TGF-β family

migration, aggregation and proliferation of mesenchymal type cells and their differentiation in to osteogenic cells

Insulin growth factor I and II (IGF):

Effects similar to TGF-β

They also stimulate proliferation of osteoblast precursors

Fibroblast growth factor (FGF) : increases proliferation of osteoprogenitor cells. promotes osteogenic differentiation

BONE LINING CELLS:

Osteoblasts flatten, when bone is not forming and extend

along the bone surface and hence the name.

They are present on periosteal as well as endosteal

surfaces.

OSTEOCYTES:

Nerve cells

Sense the change in environment and send signals that affect

response of other cells involved in bone remodelling

Maintains balance between

resorption and remodelling

Bone that forms more rapidly

shows more osteocytes.

Osteocytic lacunae

Canaliculi- narrow extension of lacunae, permits

diffusion of gases and nutrients

Maintains bone integrity and vitality

Failure of inter connecting system between osteocytes

and osteoblasts leads to sclerosis and death of bone

OSTEOCLAST:

In Greek it means “ bone and broken ’’

Morphology

Howship’s lacunae

Diameter – 50-100 um

15 to 20 nuclei ( more nuclei more

resorption)

TRAP – distinguishes from other

multinucleated giant cells

MORPHOLOGY

Extensive mitochondria except below the ruffled border

Ruffled border – deep folds

Cathepsin containing vesicles and vacuoles are present

close to ruffled border – resorptive capacity

Clear or sealing zone

FORMATION OF OSTEOCLASTS

Cells of monocyte macrophage lineage differentiate into

osteoclast by cell to cell interaction

RANKL and M-CSF are produced by osteoblasts. These are

required for formation of osteoclasts

M-CSF – proliferation and differentiation. It acts through c-fms

present on osteoclasts

RANKL- differentiation in to matured osteoclast and their

activity.

RANKL/ ODF / TRANCE( TNF related induced cytokine) /

OPGL

Formation of osteoclast

Stimulatory factors Inhibitory factors

Vitamin D3 and PTH OPG

TNF-β Calcitonin

PGE-2 OCIL

IL-1,6,8,11 IL-4,10,12,13,18

TGF- β & Interferon-ϒ

Bisphosphonates

BONE FORMATION AND FACTORS AFFECTING BONE FORMATION

Two theories have been put forward for how the bone is formed and calcified.

1st theory:Matrix vacuoles, which are produced as an outgrowth of osteoblasts or chondroblasts or odontoblasts are responsible for calcification.

2nd theory Macromolecular constituents of bone and cartilage matrix directly implicates in calcification

Factors regulating bone formation:

Platelet derived growth factor

Cationic heparin binding polypeptide

Collagen synthesis and rate of bone apposition

Acidic fibroblast growth factors and basic fibroblast

growth factor

Increases collagen synthesis

Insulin like growth factor

Increase preosteoblasts replication and stimulates collagen

synthesis

Transforming growth factor

TGF-α – resorption

TGF-β – formation

Bone morphogenetic proteins (BMPs)

during repair they are released and are required for healing

BONE RESORPTION:

Sequence of events of bone resorption: Involves 3 phases

First phase -

formation of osteoclast

Second phase-

activation of osteoclast

Third phase -

resorption of bone

Alterations in the osteoclast

Removal of hydroxyapatite

acidic environment by proton pump

Degradation of organic matrix

acid phosphatase, cathepsin B

Removal of degradation products from lacunae

endocytosis

Translocation of degraded products and extracellular release

Alterations in the osteoclast:

The osteoclasts create - Howship’s lacunae.

assumes polarity of structure and function.

The two distinct alterations are the

development of a ruffled border

sealing zone at the plasma membrane.

The cytoplasm adjacent to ruffled border is devoid of cell

organelles, contains actin microfilaments surrounded by vinculin

rings- clear zone.

When osteoclasts arrive at resorption site, they use the sealing

zone to attach themselves to the bone surface.

Removal of hydroxyapatite:

The initial phase involves the dissolution of the mineral phase –

HCl

The protons for the acid arise from the activity of cytoplasmic

carbonic anhydrase II, which is synthesized in osteoclast.

The protons are then released across the ruffled border into the

resorption zone by an ATP consuming proton pump.

This leads to a fall in pH to 2.5 to 3.0 in the osteoclast resorption

space.

Degradation of organic matrix:

Proteolytic enzymes are synthesized by osteoclasts- cathepsin

k and MMP-9.

cathepsin k is the most important enzyme in bone. It degrades

major amount of type I collagen and other non collagen proteins

MMP-9(collagenase B) - osteoclast migration.

MMP-13 -bone resorption and osteoclast differentiation.

Removal of degradation products from lacunae:

Once liberated from bone, the free organic and non organic

particles of bone matrix are taken in or endocytosed from

resorption lacunae, across the ruffled border, into the osteoclast.

These are then packed into membrane bound vesicles within

cytoplasm of osteoclast.

These vesicles and their contents pass across the cell and fuse

with functional secretory domain (FSD) a specialized region of

the basement membrane.

Then the vesicles are released by exocytosis.

Factors associated with mechanism of bone Resorption:

Interleukin 1 – IL-1α, IL-1β. It stimulates production and release of

prostaglandin PGE2

Interleukin-6 (IL-6)

Tumor necrosis factor

lymphotoxin

Gamma interferon – inhibits resorption

Colony stimulating factors

Prostaglandins and other arachidonic acid metabolites

Role of trap in bone resorption:

Synthesized as inactive pro enzyme

Bone resorption inside and outside the cell

Concentration of TRAP in serum can be assessed which indicates resorption day by day basis

BONE REMODELLING

The process by which overall size and shape of bone is

established- bone modelling.

Embryo to pre-adult period.

Rapidly formed on periosteal surface simultaneous destruction

on endosteal surface at focal points and with in the osteon.

Bone formation greater than resorption.

Bone turnover or remodelling – replacement of old bone by

new bone.

As age increases resorption exceeds

Cortical bone turnover-5% per year

Trabecular and endosteal surface – 15% per year

Coupling

The processes of bone synthesis and bone breakdown go on

simultaneously and the status of the bone represents the net result

of a balance between the two processes. This phenomenon is

called coupling.

Hormones and coupling

With the exception of calcitonin, all the hormones, cytokines, and

growth factors that act on bone, as an organ, mediate their activity

through osteoblasts.

Resorbing hormones act directly on osteoblasts, which then

produce other factors that regulate osteoclast activity.

This results in both bone formation and bone resorption being

coupled.

The coupling theory is based on the observation that once

resorption occurs, osteoblasts respond by making bone matrix.

That is, any change in resorption or formation results in

change in the other.

A hypothetical mechanism for explaining the coupling

phenomenon is that resorbing bone produces a factor that

influence the rate or extent of osteoblastic activity.

Functions of remodelling

To prevent accumulation of damaged bone by regenerating

new bone.

Allowing to respond to the changes in mechanical forces.

Mineral homeostasis.

•First the osteoclasts tunnel into surface of bone, which lasts for

3 weeks- resorb the haversian lamellae, and form a resorption

tunnel or cutting cone.

•After sometime resorption ceases and osteoclasts are replaced

by osteoblasts. These osteoblasts lay down a new set of haversian

lamellae, encircling a vessel upon a reversal line.

•This cement line is a thin layer of glycoproteins comprising

bone sialoprotein and osteopontin that acts as a cohesive

mineralized layer between the old bone and new bone to be

secreted.

•The entire area of osteon, where active formation occurs is

termed the filling cone.

•The osteoblasts get entrapped in new bone and are called

osteocytes. Fragments of lamellae from old bone haversian

systems are left behind as interstitial lamellae

Sequence of events in bone remodelling:

MEDIATORS OF BONE REMODELLING:

Parathyroid hormone

Calcitonin

Vitamin D metabolites i.e., 1, 25-dihydroxycholecalciferol

Cytokines

Prostaglandins

Growth factors

Mechanical factors

Bacterial products.

MARKERS OF BONE TURNOVER:

The markers of bone formation are: (serum markers)

•Alkaline phosphatase (total)

•Alkaline phosphatase (skeletal isoenzymes)

•Osteocalcin

•Procollagen I extension peptide

The markers of bone resorption are: (urinary markers)

•Urine calcium

•Urine hydroxy proline

•Collagen cross linking fragments

•Urine N – telopeptide

•Urine C- telopeptide

•Urine total pyridinoline

•Urine free deoxypyridinoline

Serum markers of bone resorption:

•Serum TRAP

•Serum β2 macroglobulin

Pathologies caused due to improper control of remodelling are:

•Osteoporosis

•Osteopetrosis

•Malignant bone tumors

•Inflammatory joint diseases

CONCLUSION :

The response of bone to inflammation includes bone formation as well as resorption. Thus bone loss in disease is not simply a destructive process, but results from the predominance of resorption over formation

Proper understanding of changes seen in the bone in variety of diseases will help in finding new therapeutic strategies

REFERENCES:

•Carranza’s clinical periodontology-10th edition

•Lindhe – Textbook of periodontology-5th edition

•Orban’s oral histology & embryology-13th edition

•Tencate oral histology-8th edition

•Fundamentals of Periodontics.- Thomas G. Wilson, Kenneth S. Kornman

-2nd Edition

•Biology of periodontal tissues. P. Mark Bartold and A.SampathNarayanan-1st

edition

•Periodontology 2000, Vol. 24, 2000, 99-126