Temporomandibular Joint Development

Question 1

What type of joint is the TMJ?

Answer 1

Synovial hinge and gliding joint

Question 2

Describe the TMJ when the jaw is closed.

Answer 2

Disc fits in socket, sandwiched between mandibular fossa and condyle

Question 3

Describe the TMJ when the jaw is open.

Answer 3

Disc slides forward and condyle moves forward

Question 4

Where does the TMJ form?

Answer 4

Between the mandibular (glenoid) fossa of temporal bone

And the condylar process of the dentary/mandible

Question 5

What is the TMJ equivalent in some mammals which do not involve the temporal bone?

Answer 5

Squamosal-dentary joint

Question 6

How many synovial cavities are in the TMJ?

Answer 6

2

Question 7

Why is the human temporal bone described as composite?

Answer 7

Formed by the fusion of the petromastoid, squamosal, tympanic and styloid bones

Question 8

What processes is the proximal part of the mandible divided into?

Answer 8

Condylar process

Coronoid process

Angle

Question 9

The coronoid process acts as an attachment for which muscle?

Answer 9

Temporalis

Question 10

The angle acts as an attachment for which muscle?

Answer 10

Internal pterygoid

Question 11

The condylar process acts as an attachment for which muscle?

Answer 11

External pterygoid

Question 12

What ligaments help stabilise the TMJ/jaw movement? (3)

Answer 12

TMJ ligament

Sphenomandibular ligament

Stylomandibular ligament

Question 13

What is the jaw joint in non-mammalian jawed vertebrates formed between?

Answer 13

2 cartilaginous bones:

• Quadrate
• Articular

Question 14

What are the quadrate and articular homologous to in mammals?

Answer 14

Incus and malleus in the inner ear

Question 15

Give three ways in which the TMJ can be abnormal.

Answer 15

Ankylosis

Auriculo-condylar syndrome

Under/overgrowth of condyle affects size and symmetry

Question 16

What four basic steps are required to form the TMJ?

Answer 16

Patterning (formation and shaping) of dentary

Capping of condyle with secondary cartilage

Disc formation

Endochondral ossification of cartilage

Question 17

Why might a mammal have a large coronoid process?

Answer 17

Large muscle attachment for large bite force

Question 18

What would a TGF-β2 knockout cause in a mouse and why?

Answer 18

Loss of angular process

No tendon is formed so no muscle pulling (loss of mechanical stimulation)

Question 19

What would a Pax9 mutant mouse lack and why?

Answer 19

Coronoid process due to intrinsic defects

Also hypodontia

Question 20

What happens to the secondary cartilage that forms along the anterior coronoid border?

Answer 20

Disappears before birth

Question 21

When is the condylar secondary cartilage mostly ossified in humans?

Answer 21

Half-way through gestation

Question 22

Which TMJ synovial cavity forms first?

Answer 22

Upper/superior

Question 23

Briefly describe the formation of the disc. (2)

Answer 23

Upper cavity forms first

High levels of lubricin help disc to lift off condylar process

Question 24

Why is fetal jaw movement important in disc formation?

Answer 24

Movement stimulates cells to change shape from rounded chondrocytes to fibroblast-like

So disc can lift off condylar process

Question 25

How does loss of Indian Hedgehog affect the TMJ?

Answer 25

Reduction in condylar cartilage and failure in disc formation

Question 26

What syndrome are Tcof1 mutant mice used to investigate?

Answer 26

Treacher-Collins syndrome

Question 27

What is different about the TMJ in Tcof1 mutant mice?

Answer 27

Hypoplastic condyle with ossified disc (painful articulation)

Question 28

Why do condyle defects affect the developing mandibular fossa?

Answer 28

As condyle and temporal/squamosal tissues form they communicate to coordinate growth Condyle disruption results in failure of squamosal bones to fuse and hence no fossa

Question 29

In the condylar secondary cartilage, where is most of the proliferation?

Answer 29

In progenitor cells in polymorphic cell layer (beneath the articular layer)

Question 30

In secondary cartilage, where is Runx2 expressed?

Answer 30

Prehypertrophic cartilage

Question 31

If Runx2 is knocked out, what does that mean for the secondary cartilage?

Answer 31

No hypertrophic cartilage No bone/ossification

Question 32

What happens if there is too much proliferation in the condylar secondary cartilage?

Answer 32

More growth and enlarged condyle

Question 33

What happens if there is too much differentiation in the condylar secondary cartilage?

Answer 33

Depleted pool of progenitor cells so shortened condyle (achondroplasia)

Question 34

What is thanatophoric dysplasia?

Answer 34

Asymmetrical growth of condyles

Question 35

What happens to the condylar process if there is insufficient mechanical stress?

Answer 35

Reduced/failed endochondral ossification