OE L17 Alveolar Bone and Cementum Matrix Flashcards
What is the main mechanism of bone formation in the mandible and maxilla?
Intra-membranous ossification.
What are the 3 types of bone related to the oral cavity?
- Cortical bone
- Cancellous (trabecular) bone
- Bundle bone
What is the main mechanism of bone formation in the mandibular condyles?
Endochondral ossification.
What are the 2 types of lamellar bone called?
- Cortical/compact bone
- Cancellous/trabecular bone
Describe cortical bone.
- Dense and strong
- Low porosity
- Provides protection from external environement and trauma
- Features osteons and haversian cancals
- Surrounded by concentric rings of collagen fibres
Describe cancellous bone.
- Higher porosity, less strong
- Reduced mineral content
- Degree of elasticity, withstand mechanical forces
- Has lamellae with Haversion systems in large trabeculae
- Contains bone marrow spaces
Describe the osteons in cortical bone.
- Osteons are cylindircal structures made of concentric layers (lamellae) surrounding a central Haversian canal
- Osteocytes embedded between concentric layers in spaces called lacunae
- Osteons connected to eachother by Volkmann’s canals
- Osteocytes connected to eachother by canaliculi
What is the function of osteocytes?
- Exchange of nutrients and metabolic waste
- Maintain bone formation and calcium homeostasis
Describe bundle bone.
- Immature bone
- Weakest and least mineralsied bone in alveolar process
- High remodelling capacity
- Similair to woven bone
Where are cortical, cancellous and bundle bone found in the oral cavity?
- Cortical bone: outer and inner alveolar plates
- Cancellous bone: central alveolar process
- Bundle bone: lamina dura
What is woven bone?
- Immature bone present during development and after fracture
- Poorly mineralised, low density, low mechanical strength
- Produced when osteoblasts rapidly produce osteoid
Later replaced by lamellar bone in foetal tissues.
How does rate of formation influence levels of bone mineralisation?
Quicker bone is formed, less mineralised.
Bundle bone quickest formed, weakest.
Cancellous bone formed more slowly, more mineralised.
Cortical bone formed slowest, most mineralised.
What are the 3 fundamental bone cells?
- Osteoblasts
- Osteocytes
- Osteoclasts
Describe the origin of osteoblasts and their function.
- Formed by differentiation of bone marrow derived mesenchymal stem cells
- Responsible for overall bone formation
- Produce osteoid (bone matrix)
What are osteocytes?
Mature osteoblasts which become embedded within the bone.
Describe the origin of osteoclasts and their function.
- Formed by differentiation of bone marrow derived hematopoietic stem cells
- Responsible for overall bone resoption
What are the 2 stages of bone formation? Describe them.
Osteoinduction: mesenchymal stem cells migrate to required site and proliferate
Osteoconduction: once stem cells are present in adequate numbers, they differentiate into osteoprogenitor cells and then mature to become osteoblasts
Which transcription factor causes osteoblast formation?
RUNX2 is expressed by osteoprogenitor cells. Which in turn allows osteoblast markers to be expressed and therefore osteoblasts form.
What induces and inhibits osteoconduction?
Induced by: BMPs, VEGF
Inhibited by: PDGF, bFGF
Osteoblasts secrete osteoid- what is this?
The protein mixture that forms the organic matrix of bone.
Describe the series of events for osteoid secretion.
- First proteins secreted are inhibitors/regulators of mineralisation e.g. hyaluronan and versican
- Fibronectin, tenascin and thrombospondin secreted and have roles in matrix formation and cell adhesion
- Matrix metalloproteinases secreted to remove the ECM components preventing mineralisation
- Matrix begins to assemble that usually associated with bone, components include type 1 collagen, decorin, biglycan and bone glycoproteins
Describe the series of events for bone minerlisation.
- CS decorin regulates collagen fibril diamter and orientation
- Type I collagen acts as the structural framework for mineralisation
- Chonroitin sulfate chains attract calicum and phopshate
- Bone glycoproteins are acidic so can bind to HAP and regulate HAP crystal growth
What is the composition of mineralised bone?
- 70% mineral content: hydroxyapatite and impurities e.g. fluoride, magnesium
90% of the organic matrix is collagen (I and III) - 30% non-collagenous matrix: contains chondroitin sulphate-subsitiuted decorin and biglycan, bone sialoprotein, osetopontin and osteocalcin
What is the composition of mineralised cementum?
- 50% mineral content: hydroxyapatite and impurities e.g. fluoride, magnesium
90% of the organic matrix is collagen (I and III) - 30% non-collagenous matrix: contains chondroitin sulphate-subsitiuted decorin and biglycan, bone sialoprotein, osetopontin, osteocalcin, osteonectin, tenascin and fibronectin
What is the major similarity between alveolar bone and cementum composition?
Both have a non-collagenous matrix component of 30%.
If cementum and bone have similair components, why is cementum less mineralised than bone?
Cementum is less mineralised because:
- organisation of matrix is different
- different position of collagen fibres
- slower deposition rate
- lacks vascularity and innervation
Bone is constantly remodelled. What happens if this remodelling becomes unbalanced?
In diseased states osteoclast acitivity is greater than osteoblast activity.
This loss of balance causes greater bone resorption e.g. periodontitis
What is the mechanism for osteoclast differentiation?
- Osteoblasts express cell surface receptor RANKL
- RANKL binds to RANK receptor on haematopoietic stem cells surfaces
- Binding induces stem cells to differentiate and fuse to form large multi-nucleated mature osteoclasts
How does osteoprotegerin affect osteoclast acitivity?
- Osteoprotegerin is produced by osteoblasts and mesenchymal stem cells
- Prevents RANKL-RANK interaction
- Haematopoietic stem cell differentiation inhibited
- Osteoclast number reuced
Describe the action of osteoclasts.
Removal of mineral content in bone and breakdown of the matrix.
What is the osteoclast resoprtion cycle?
- Migration to resorption site
- Attach to bone and form a sealing zone and ruffled border
- HAP dissolution through HCl secretion from ruffled border
- ECM degradation by cysteine proteinases and MMPs
- Apoptosis of osteoclast or return to non-resorbing state
How do osteoclasts dissolve mineral in bone?
- Chloride ions released from osteoclasts
- Carbonic anhydrase II catalyses reaction of carbon dioxide and water to form carbonic acid
- Carbonic acid dissocaites into H+ ions
- Hydrogen ions + chlorine ions = HCl
Does alveolar bone have a high turnover rate?
Yes, alveolar bone has a high turnover rate.
