Molecular Aspects of the TMJ Flashcards
Give some details on the TMJ
A hinge and gliding joint and is the most constantly used joint in the body.
Found in all mammals.
It consists of the disc and involves the muscles surrounding the joint.
What happens to the disc and the condyte (round end of the lower jaw) during a closed and an open jaw?
Closed = disc fits in the socket when the jaw is closed, the condyte fits in the socket when the jaw is closed.
Open = disc slides forward as the jaw opens, condyte moves forward as the jaw opens.
Where does the TMJ form?
Where does the disc form and what is it surrounded by?
What cavity does the disc have?
TMJ forms between the glenoid fossa of the temporal bone and the condylar process of the denture (mandible).
A disc forms between these two skeletal elements, surrounded by synovial fluid.
Cup shape in upper jaw and then a ball sitting at the bottom.
The disc has a synovial cavity present.
The temporal bone:
What forms part of this?
What 3 processes is the proximal part divided into?
What is the only bone found in the lower jaw?
The glenoid or mandibular fossa part of the temporal bone which is a composite bone compromised of the petromastoid, squamosal, tympanic and styloid.
The proximal part of the human mandible is divided into the coronoid, condylar and angle process.
Dentary bone is the only bone in the lower jaw.
What type of muscle is at the coronoid process and the angle process?
Coronoid - jaw closing muscles
Angle - jaw opening muscles
The articulation with the temporal bone occurs at the —-, while the angle and coronoid act as a —- attachment site.
Condyle, muscle.
What 3 things do ligaments stop from happening?
Overopening, overbending and from closing with too much force.
What is the TMJ unique to?
What do non-mammalian jawed vertebrates use?
Unique to mammals.
Use a jaw joint formed from two cartilaginous bones (the quadrate and articular).
What are the 3 main defects associated with the TMJ?
- Ankylosis (fusion) of TMJ = fusion of upper and lower jaw
- Auriculo-condylar syndrome
- Under and overgrowth of condylar (leading to defects in jaw size)
What are the 4 steps to making a jaw articulation in the embryo?
- Patterning of the dentary (intramembranous ossification) - needed to turn cartilage into bone
- Capping of the bone with secondary cartilage
- Formation of a disc
- Transformation of the cartilage into bone (endochondral ossification) for jaw growth
Which bone is the main variation in the dentary?
The coronoid as this is needed for biting.
Patterning of the processes of the dentary:
Give details on this
The three processes appear to be under separate genetic control.
This allows a greater flexibility and ability to cope with changing diets and lifestyle.
In mammals, the three processes form early during development and then differences in growth determine their final shape.
Defects in the patterning of the dentary can be caused mechanical factors or intrinsic defects in the forming bone,
Secondary cartilage:
- When do they form?
- Where do they form?
- What does it help with?
- Form after the first wave of bone formation
- It forms at the condyle. The cartilage is largely replaced by bone half way through gestation, except at the articulation site which remains cartilaginous. It also forms along the anterior coronoid border
- Cartilage is added to help with articulation
What happens if there is a loss of BMP signalling?
Failure of secondary cartilage formation leading to failure in articulation.
Where does the development of the disc occur?
On top of the condylar cartilage.
Why are there high levels of lubricin (a glycoprotein) expressed during disc formation?
To separate the disc from the condylar
What is fetal jaw movement (mechanical stimulation) important for?
Important for formation of the disc and condylar cartilage.
Gives the correct morphology of the TMJ.
What happens in these weeks of disc development?
9-11?
10-12?
11-13?
9-11 = first evidence of cavity formation. Disc separation
10-12 = clear inferior cavity
11-13 = upper and lower synovial cavities around disc form
Why do many mammals without teeth have no disc in the TMJ?
In the absence of muscle action, the disc does not lift off the condylar.
Loss of the gene ‘indian hedgehog’ leads to what?
Reduction in the condylar cartilage and a failure in disc formation.
What is ankylosis and what causes it?
It is abnormal stiffening and immobility of a joint due to fusion of the bones.
Defects in the formation of the disc leads to this fusion.
What do condylar defects affect?
When it is not forming correctly, it affects the upper jaw.
The orientation and growth patterns of the condylar cartilage are one of the important determinants of co-ordinated craniofacial growth.
The secondary cartilage acts as a growth centre extending the —- —- of the dentary?
Explain this
Bony processes.
Within the condyle chondrocytes pass through proliferative and hypertonic zones before being replaces by osteoblasts and bone matrix.
Explain the regions of cells going from the disc cells down to the hypertrophic chondrocytes?
In the outer regions the chondrocytes are flattened, towards the centre rounded. As they move inferiorly they become hypertrophic. Most of the proliferation is in the progenitor cells in the polymorphic cell layer beneath the articular layer.
What type of collagen is expressed in the condylar cartilage?
In the hypertrophic cells?
Type 2
Type 10 (the marker for bone).
What is the importance of balance between proliferation and differentiation?
Too high proliferation leads to more growth and enlarged condyles (jaw would continue to grow).
Too high differentiation depletes the pool of progenitor cells and leads to shortened elements.
Balance is disrupted in a number of human disorders.
Endochondral ossification: Transformation of cartilage into bone is determined by the amount the jaw is used. The role of mechanical stress is what?
Mechanical stress is thought to be important for ossification of the condylar process.
If there is insufficient mechanical stress from jaw movement/mastication, endochondral ossification is reduced or fails completely.
What are some conclusions of the TMJ?
- Unique to mammals
- Condylar cartilage caps the condyle of the dentary to create an articulation point with the upper jaw
- A disc forms associated with the condylar cartilage
- Formation of synovial fluid creates a superior and inferior cavity on either side of the disc
- Development and growth of the condyle is a complex process involving control of proliferation, differentiation and invasion of bone.
- Both intrinsic and external factors play a role in regulating these processes.
What is the name of the articulation site into which the mandibular condylar inserts?
Mandibular fossa
Name some of the bones that make up the temporal bone
Tympanic, styloid, squamosal and petromastoid
Where in the mammalian body are the homologous bones that form the jaw articulation in non-mammals?
In the inner ear
How can you tell an animal has a big bite force from the dentary bone?
It has a large coronoid process for lots of muscle attachment to get the big bite.
How can you change the shape of the dentary during development?
Muscles shape the bones due to the movement of the jaw (muscle attachment)
How does the disc form?
Condenses over condylar, get an upper synovial cavity and lower cavity, stretching of disc cells in fibroblast morphology. Lower synovial cavity as it lifts off and then we get stretching of the disc cells into a fibroblast morphology
Name a gene involved in gene formation
Indian hedgehog
Describe the different cell types in the adult condylar
Small proliferating cells at the tip, then chondrocytes, hypertrophic chondrocytes and then bone
What drives endochondral ossification in the condylar?
Eating
