3. CONTENTS
• INTODUCTION
• CLASSIFICATION OF OSSIFICATION
• PROCESSES DURING OSSIFICATION
• FACTORS AFFECTING BONE FORMATION
• BONE RESORPTION
• FACTORS AFFECTING BONE RESORPTION
• BONE REMODELLING
• CONCLUSION
• REFERRENCES
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4. INTRODUCTION
Human skeleton is made up of 206 bones
out of which 28 bones form the skull and
facial skeleton.
Formation of bony skeleton begins in
second month of development.
Postnatal bone growth occurs until early
adulthood. Bone remodeling and repair
are lifelong processes.
Up to about week 8, fibrous membranes
and hyaline cartilage of fetal skeleton are
replaced with bone tissue. Ossification is
the term used to describe process of bone
formation by deposition of calcium in the
fetal hyaline cartilage and mesenchymal
tissue.
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5. CLASSIFICATION OF OSSIFICATION
OSSIFICATION
PRIMARY OSSIFICATION SECONDARY OSSIFICATION
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Primary Ossification or Primary angiogenic ossification of Krompecher :
Bone is synthesized de novo by osteoblasts differentiated from
mesenchymal cells found in the adventitia of small vessels – in human it is
not seen.
Secondary ossification : Bone replaces a former tissue type, the former
tissue forms into a shape producing, load-bearing tissue
6. SECONDARY OSSIFICATION
INTRAMEMBRANOUS OSSIFICATION ENDOCHONDRAL OSSIFICATION
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INTRAMEMBRANOUS OSSIFICATION: Osteoblasts differentiate
from mesenchymal cells and replace the preexisting connective
tissue. Typical in flat bones. The Frontal Bone, Parietal Bone,
Part of Occipital and Temporal Bone, Clavicle, Mandible
ENDOCHONDRAL OSSIFICATION: Load-bearing cartilage
replaced and rebuild into bone. Characteristic for long bones. All
the bones of the body except the ones mentioned above are
formed in this way.
7. INTRAMEMBRANOUS OSSIFICATION
STAGES
1. FORMATION OF MATRIX AND ORGANIC COMPONENTS.
• Osteoblast cluster and secrete organic matrix components including
collagen fibers.
• Mineralization of matrix through crystallization of calcium salts.
• Differentiation of osteoblats into osteocytes.
2. OSSIFICATION CENTER DEVELOPMENT.
• Bone grow outward from this center called spicules.
• Osteoblast formation continue from mesenchymal cells.
3. SPONGY BONE FORMATION
• Woven bone or spongy bone is formed when osteoid is laid down around
blood vessels, resulting in trabeculae.
• Outer layer of woven bone forms periosteum.
• Lamellar bone replaces woven bone, and red marrow appears.
• Subsequent remodelling around trapped blood vessels resulted to
compact bone.
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13. ENDOCHONDRAL OSSIFICATION
STAGES
Endochondrial ossification involves replacement of cartilage by
bone and forms most of the bones of the body
THE FIRST STEP in endochondrial ossification is the development of
the cartilage model.
• Begins in the second month of development
• Uses hyaline cartilage “bones” as models for bone construction
• Requires breakdown of hyaline cartilage prior to ossification
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14. Development of Cartilage
model
• Mesenchymal cells form a
cartilage model of the bone
during development
Growth of Cartilage model
• In length by chondrocyte cell
division and matrix formation (
interstitial growth)
• In width by formation of new
matrix on the periphery by
new chondroblasts from the
perichondrium (appositional
growth)
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15. • Formation of bone collar
• Cavitation of the hyaline: cartilage cells in mid-
region burst and change pH triggering
calcification and chondrocyte death
• Invasion of internal cavities by the periosteal bud,
and spongy bone formation
• Formation of the medullary cavity; appearance of
secondary ossification centers in the epiphyses
• Ossification of the epiphyses, with hyaline
cartilage remaining only in the epiphyseal plates
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16. Development of Primary
Ossification Center
• Perichondrium lays down
periosteal bone collar
• Nutrient artery penetrates
center of cartilage model
• Periosteal bud brings
osteoblasts and osteoclasts
to center of cartilage model
• Osteoblasts deposit bone
matrix over calcified
cartilage forming spongy
bone trabeculae
• Osteoclasts form medullary
cavity
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17. Development of Secondary
Ossification Center
• Blood vessels enter the
epiphyses around time of
birth
• Spongy bone is formed
but no medullary cavity
Formation of articular
cartilage
• Cartilage on ends of bone
remains as articular
cartilage and epiphysial
plate.
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19. FACTORS AFFECTING BONE FORMATION
• Platelet derived growth factor
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
• Nutrition
Adequate levels of minerals and vitamins like calcium and phosphorus for
bone growth; Vitamin C for collagen formation and Vitamins K and B12 for
protein synthesis.
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20. BONE RESORPTION
INVOLVES 3 PHASES
First phase -
Formation of osteoclast
Second phase -
Activation and alteration of osteoclast
Third phase -
Resorption of bone
FORMATION OF OSTEOCLAST
The monocyte phagocytic system is the precursor of osteoclasts. Osteoclast
formation requires the presence of RANKL (receptor activator of nuclear
factor κβ ligand) and M-CSF (macrophage colony-stimulating factor). These
membrane-bound proteins are produced by neighbouring stromal cells and
osteoblasts, thus requiring direct contact between these cells and osteoclast
precursors.
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21. ALTARATION OF OSTEOCLAST
Activated osteoclasts assume polarity of structure and function.
- The two distinct alterations are the
• Development of a ruffled border
• Sealing zone at the plasma membrane.
- The osteoclasts have a ruffled border. The cytoplasm adjacent
to ruffled border is devoid of cell organelles, contains actin
microfilaments surrounded by vinculin ( an actin binding
protein)rings known as clear zone.
- When osteoclasts arrive at resorption site, they use the sealing
zone to attach themselves to the bone surface.
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22. REMOVAL OF HYDROXYAPATITE
The initial phase involves the
dissolution of the mineral by HCl
secreted by osteoclasts.
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.
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23. DEGRADATION OF ORGANIC MATRIX:
• Proteolytic enzymes are synthesized by osteoclasts- cathepsin k and MMP-
9, MMP-13.
• Cathepsin k is the most important enzyme in bone. It degrades major
amount of type I collagen & 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.
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24. 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
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25. BONE REMODELLING
The process by which overall size and shape of bone is established is called bone
modelling.
REASONS OF REMODELLING
• To prevent accumulation of damaged bone by regenerating new bone.
• Allowing to respond to the changes in mechanical forces.
• Mineral homeostasis.
MECHANISM
•First the osteoclasts tunnel into surface of bone 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
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