Molecular Aspects of the TMJ

Question 1

Give some details on the TMJ

Answer 1

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.

Question 2

What happens to the disc and the condyte (round end of the lower jaw) during a closed and an open jaw?

Answer 2

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.

Question 3

Where does the TMJ form?
Where does the disc form and what is it surrounded by?
What cavity does the disc have?

Answer 3

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.

Question 4

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?

Answer 4

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.

Question 5

What type of muscle is at the coronoid process and the angle process?

Answer 5

Coronoid - jaw closing muscles

Angle - jaw opening muscles

Question 6

The articulation with the temporal bone occurs at the —-, while the angle and coronoid act as a —- attachment site.

Answer 6

Condyle, muscle.

Question 7

What 3 things do ligaments stop from happening?

Answer 7

Overopening, overbending and from closing with too much force.

Question 8

What is the TMJ unique to?

What do non-mammalian jawed vertebrates use?

Answer 8

Unique to mammals.

Use a jaw joint formed from two cartilaginous bones (the quadrate and articular).

Question 9

What are the 3 main defects associated with the TMJ?

Answer 9

• Ankylosis (fusion) of TMJ = fusion of upper and lower jaw
• Auriculo-condylar syndrome
• Under and overgrowth of condylar (leading to defects in jaw size)

Question 10

What are the 4 steps to making a jaw articulation in the embryo?

Answer 10

1. Patterning of the dentary (intramembranous ossification) - needed to turn cartilage into bone
2. Capping of the bone with secondary cartilage
3. Formation of a disc
4. Transformation of the cartilage into bone (endochondral ossification) for jaw growth

Question 11

Which bone is the main variation in the dentary?

Answer 11

The coronoid as this is needed for biting.

Question 12

Patterning of the processes of the dentary:

Give details on this

Answer 12

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,

Question 13

Secondary cartilage:

• When do they form?
• Where do they form?
• What does it help with?

Answer 13

• 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

Question 14

What happens if there is a loss of BMP signalling?

Answer 14

Failure of secondary cartilage formation leading to failure in articulation.

Question 15

Where does the development of the disc occur?

Answer 15

On top of the condylar cartilage.

Question 16

Why are there high levels of lubricin (a glycoprotein) expressed during disc formation?

Answer 16

To separate the disc from the condylar

Question 17

What is fetal jaw movement (mechanical stimulation) important for?

Answer 17

Important for formation of the disc and condylar cartilage.

Gives the correct morphology of the TMJ.

Question 18

What happens in these weeks of disc development?
9-11?
10-12?
11-13?

Answer 18

9-11 = first evidence of cavity formation. Disc separation

10-12 = clear inferior cavity

11-13 = upper and lower synovial cavities around disc form

Question 19

Why do many mammals without teeth have no disc in the TMJ?

Answer 19

In the absence of muscle action, the disc does not lift off the condylar.

Question 20

Loss of the gene ‘indian hedgehog’ leads to what?

Answer 20

Reduction in the condylar cartilage and a failure in disc formation.

Question 21

What is ankylosis and what causes it?

Answer 21

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.

Question 22

What do condylar defects affect?

Answer 22

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.

Question 23

The secondary cartilage acts as a growth centre extending the —- —- of the dentary?
Explain this

Answer 23

Bony processes.

Within the condyle chondrocytes pass through proliferative and hypertonic zones before being replaces by osteoblasts and bone matrix.

Question 24

Explain the regions of cells going from the disc cells down to the hypertrophic chondrocytes?

Answer 24

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.

Question 25

What type of collagen is expressed in the condylar cartilage?
In the hypertrophic cells?

Answer 25

Type 2

Type 10 (the marker for bone).

Question 26

What is the importance of balance between proliferation and differentiation?

Answer 26

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.

Question 27

Endochondral ossification: Transformation of cartilage into bone is determined by the amount the jaw is used. The role of mechanical stress is what?

Answer 27

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.

Question 28

What are some conclusions of the TMJ?

Answer 28

• 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.

Question 29

What is the name of the articulation site into which the mandibular condylar inserts?

Answer 29

Mandibular fossa

Question 30

Name some of the bones that make up the temporal bone

Answer 30

Tympanic, styloid, squamosal and petromastoid

Question 31

Where in the mammalian body are the homologous bones that form the jaw articulation in non-mammals?

Answer 31

In the inner ear

Question 32

How can you tell an animal has a big bite force from the dentary bone?

Answer 32

It has a large coronoid process for lots of muscle attachment to get the big bite.

Question 33

How can you change the shape of the dentary during development?

Answer 33

Muscles shape the bones due to the movement of the jaw (muscle attachment)

Question 34

How does the disc form?

Answer 34

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

Question 35

Name a gene involved in gene formation

Answer 35

Indian hedgehog

Question 36

Describe the different cell types in the adult condylar

Answer 36

Small proliferating cells at the tip, then chondrocytes, hypertrophic chondrocytes and then bone

Question 37

What drives endochondral ossification in the condylar?

Answer 37

Eating