Alveolar bone Flashcards
What is alveolar bone?
It is a mineralised connective tissue that supports and protects the teeth
What percentage of alveolar bones net weight is inorganic material?
60%
What percentage of alveolar bones net weight is organic material?
25%
What percentage of alveolar bones net weight is water?
15%
What is the inorganic material of alveolar bone mainly made up of?
Hydroxyapatite crystal
What are hydroxyapatite crystals and what do they do?
They are crystals that impregnate and surround collagen fibres providing rigidity and resistance to compression
What is the organic component of alveolar bone made upon?
90% type I collagen
10% is a complex of non-collagenous proteins like glycoprotein (osteonectin and osteopontin), proteoglycans, osteocalcin, growth factors and serum proteins
Which cell produces all these proteins?
Osteoclasts
How much of the organic metric is made up of collagen?
90%
What is most of the collagen in the organic matrix regarded as?
intrinsic collagen secreted by osteoblasts
When is collagen considered extrinsic?
When it is inserted as Sharpey’s Fibres formed by adjacent fibroblasts
What is the major type of collagen found in the matrix?
Type I
But trace amounts of type III and V may also be present
What form does the mineral in alveolar bone take? And what is its relative width and thickness?
Mineral is found in the form of needle like crystallites
50 nm wide
8nm thick
Variable length
Where are the crystallites distributed in the bone?
Distributed within the space between and on the surface of the collagen fibrils
What are the 4 stages of bone remodelling?
Resorption
Reversal
Formation
Resting
(Basically bone removed then reformed)
What does bone remodelling allow the alveolar bone to do?
Allows the bone to remodel and adapt to changing functional demands placed upon it
Why is there no change in the overall shape of bone even though resorption is occurring?
As resorption is occurring at the same rate as bone formation so overall shape in bone is unchanged
How is the rate of formation and resorption co ordinated?
The actions of the various cell types is coordinated
Osteoblasts form bone
Osteoclasts resorb bone
And the combined action of osteoblasts and osteocytes have the ability to detect mechanical stresses the bone is subjected to
What happens to bone during the resorption stage?
Recruitment,migration and activation of osteoclasts causes resorption
What happens to bone during the reversal stage?
Resorption ends and osteoclasts disappear by apoptosis or migration
This stopping of resorption is characterised by a ‘reversal line’
What happens to bone during the formation stage?
Osteoblast recruitment, migration, differentiation and formation of new bone occurs in the resorption site
What happens to bone during the resting stage?
Formation of bone stops following mineralisation
Surface is lined by a flattened layer of cells (bone lining cells)
What are the 4 main cell types in bone?
Bone lining cells
Osteoclasts
Osteocytes
Osteoblasts
Describe the structure of bone lining cells and where they are found
Are seen as a layer of flattened cells when bone surfaces are in the resting stage
What is the role of osteoclasts and how does their structure help them do this?
Osteoclasts resorb bone
They lie in resorption concavities (called Howships’s lacunae) on the alveolar bone
What are Howship’s lacunae?
Resorption concavities where osteoclasts lie on the surface of alveolar bone
How many stages does resorption occur in and what are those stages?
Resorption occurs in two stages
- Mineral phase is removed
- Organic matrix is then removed
In what pH is the mineral phase dissolved (in resorption) and how is this pH achieved ?
Low pH (acidic conditions) is required for dissolving the mineral phase To provide this low pH the osteoclasts secrete protons across the ruffled borders
How is the organic matrix dissolved in resorption?
Organic matrix exposed in the resorbing lacuna is degraded by enzymes (mainly cathepsin K)
What is the main enzyme that dissolves the organic matrix?
Cathepsin K
What type of border do osteoclasts have when they are actively degrading bone?
A ruffled border adjacent to the bone surface its degrading
If the osteoclasts have a ruffled border then what are they doing?
Actively degrading the bone it is adjacent to
What type of cells are osteoblasts?
Specialised connective tissue cells
Where are osteoblast cells predominantly found?
They are found in a layer predominant on bone surfaces where there is active bone formation
Which cells do osteoblasts keep in contact with?
Osteoblast cells
Underlying osteocytes
What do osteoblasts secrete?
They secrete formative components of bone
They also secrete molecules that control their own activity and molecules that have a controlling influence in actiavting osteoclasts
Give 4 features of osteoblasts
hint its origin, where it is found in bone what it secretes etc
- Mesenchymal in origin
- Lie on the surface of forming bone
- Secretes and mineralises organic matrix
- Incorporated into the bone matrix as osteocytes
Give 5 features of osteocytes
hint its origin, where it is found in bone, what it does and what other cells it communicates with etc
- Mesenchymal in origin
- Lies within the bone and is surrounded by bone matrix
- It’s the most abundant cell type
- Communicates with each other and with osteoblasts vis fine canaliculi
- They detect strains in bone
Give 5 features of osteoclasts
hint its origin, what they do and how they do it, where they are found etc
- Haemopoietic in origin
- Are multi-nucleated giant cells that resorb bones
- They are found with ruffled borders adjacent to the bone surface they are about to resorb
- They are rich in the acid phosphatase
- The ruffled borders pump acid to dissolve bone mineral and enzymes to degrade collagenous matrix
What does it mean to be mesenchymal in origin?
It means these cells are derived form mesenchymal (or ectomesenchymal) stem cells
Where are mesenchymal (or ectomesenchymal) stem cells found?
They reside in bone marrow and in a region of proliferating cells adjacent to the osteoblast layer in the periosteum
Which cells in the alveolar bone are mesenchymal in origin?
Osteoblasts
Osteocytes
What does it mean to be haemopoietic in origin?
Means those cells are derived from the mononuclear haemopoietic system
So the mononuclear precursors fuse to give rise to multi-nucleated osteoclast cells
Which cells in the alveolar bone are haemopoietic in origin?
Osteoclasts
What are the 2 catagories mature bone can fall under?
Compact (cortical)
Cancellous
What determines which category bone falls under?
The bones density
What does compact bone form?
Forms a dense, sold mass
What does cancellous bone form?
A lattice arrangement of individual bony trabeculae that surround marrow
Which category of bone gives maximum strength at minimum skeletal weight?
A combination of both cancellous and compact
What percentage of bones in the body are of compact nature?
80%
What percentage of bones in the body are of cancellous nature?
20%
There is a thin layer of compact bone that lines the tooth socket what does that do?
It gives attachment to the principle fibres of the periodontal ligaments
How which is one layer of bone lamellae?
3-5 micrometres thick
What two major patterns are lamellae arranged in compact bone?
Circumferential Lamellae
Haversian systems
Where are circumferential lamellae found and how are they arranged?
They are found on both external (periosteal) and internal (endosteal) surfaces
They are arranged in parallel layers completely surrouding the bony surfaces
Where are the Haversian systems found and how are they arranged?
Found deep to the circumferential
Lamellae are arranged as small concentric layers around a neurovascular canal.
The Haversian system itself is the central canal together with 20 concentric lamellae
What is a consequence of remodelling on lamellae structure?
Fragments of pervious Haversian systems may be present as well as old circumferential lamellar
What do lamellae form in cancellous bone?
They form trabeculae which surround the marrow space
What is origin of the marrow in young bone and what does this mean?
Haemopoietic
It contains stem cells of both mesenchymal type and blood cell lineage
This means the cells are able to differentiate into either osteoblasts or osteoclasts
What are Sharpey’s fibres?
They are extrinsic fibres that insert into the cribriform bone plate.
Where are cribriform bone pates derived from…?
The principal fibres of the periodontal ligament
The fibres that enter bone P_________ to the surface are less n_________ but t_______ than those at the C________ surface
Perpendicular
Numerous
Thicker
Cementum
How mineralised are the embedded fibres?
The degree of mineralisation of these fibres varies
What are trans alveolar fibres?
Sharpey’s fibres entering the bone in the mesio-distal plane may pass straight through to become continuous with similar fibres from the root to the adjacent fibre. These are trans alveolar fibres.
What are surfaces where active bone formation covered in?
A layer of newly deposited, unmineralised bone matrix called osteoid
What are osteoids
Layers of newly deposited unmineralised bone matrix
What are osteoids produced by?
Osteoblasts
At what thickness does mineralisation commence?
When a thickness of 5-10 micrometres is reached mineralisation commences along a linear mineralising front
What do osteoids consists of?
Type I collagen fibrils arranged parallel to the bone surface, embedded in a complex ground substance of proteoglycans, glycoproteins and other protein substances
What is the lag stage (in relation to osteoids)?
There is a lag phase before the deeper layer of the osteoid has matures sufficiently to undergo mineralisation
What is the resting/ mineralisation line?
A smooth regular line
Often several are running in parallel to each other
What does the reversal line mark?
It marks the bone surface when, at some point in the past, when resorption ‘reversed’ to formation
The functional load on the area of bone changes
A smooth regular line often several are running in parallel to each other are called?
The resting/mineralisation line
A more irregular line with scalloped appearance is called?
The reversal line
