This document summarizes bone formation and resorption. It classifies bones based on shape, development, and microscopic structure. It describes the composition of bone including bone cells like osteoblasts, osteocytes, and osteoclasts. Bone formation is influenced by growth factors and hormones. Bone resorption involves osteoclasts acidifying the bone surface and releasing enzymes to degrade the organic matrix. Bone remodeling maintains bone strength through coupled bone resorption and formation mediated by hormones and growth factors.

1. BONE FORMATION AND
RESORPTION
PRESENTED BY,
Dr. Dandu Sivasai Prasad Reddy
1st yr PG
Mamata dental college

2. 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

3. •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

4. INTRODUCTION

5. CLASSIFICATION OF BONES:
Shape development histology
Long flat irregular endochondral Intramembranous sutural mature immature
compact cancellous
short

6. CLASSIFICATION…
 Based on shape:
Long bones
Short bones
Flat bones
Irregular bones

7. BASED ON SHAPE
Limbs Carpals
Tarsals
Vertebrae Skull bones, Ribs
Sternum, Scapula

8. CLASSIFICATION…
 BASED ON DEVELOPMENT:
ENDOCHONDRAL BONES:
Intra cartilaginous or endochondral bone
formation or indirect bone formation:

9. CLASSIFICATION…
 INTRAMEMBRANOUS BONES:
Intramembranous ossification/direct bone
formation
 SUTURAL BONES:
Growing face and skull.

10. CLASSIFICATION…
 BASED ON MICROSCOPIC STRUCTURE:

11. COMPOSITION OF BONE:
Bone
33% organic
28% collagen
5% non
collagenous
proteins
67% inorganic Hydroxyapatite

12. 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.

13. Osteoblasts :
Derived from osteoprogenitor cells
Periosteum serves as important reservoir .
Morphology :
 basophilic
 cuboidal or slightly elongated cells
contain prominent bundles of actin, myosin
BONE CELLS:

15. Formation :
Pleuripotent stem cells
IOPC’S
DOPC’S
BMP’s ,GF’s
Osteoblasts

16. 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.

17. 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.

18.  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

19.  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

20. 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.

21. 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.

22. 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

23. 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

24. 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

26. FORMATION OF OSTEOCLASTS

27. 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

28. 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

29. 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

30. 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

31. 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

32. 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

33.  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

34. 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.

36. 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.

37. 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.

38. 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.

39. 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

40. 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

41. 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.

42. 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.

43. 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.

44. 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.

45. Functions of remodelling
To prevent accumulation of damaged bone by regenerating
new bone.
Allowing to respond to the changes in mechanical forces.
Mineral homeostasis.

46. •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.

47. •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

48. Sequence of events in bone remodelling:

49. MEDIATORS OF BONE REMODELLING:
 Parathyroid hormone
 Calcitonin
 Vitamin D metabolites i.e., 1, 25-dihydroxycholecalciferol
 Cytokines
 Prostaglandins
 Growth factors
 Mechanical factors
 Bacterial products.

50. MARKERS OF BONE TURNOVER:
The markers of bone formation are: (serum markers)
•Alkaline phosphatase (total)
•Alkaline phosphatase (skeletal isoenzymes)
•Osteocalcin
•Procollagen I extension peptide

51. 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

52. 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

53. 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

54. 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