Post on 07-May-2015
description
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