pre-natal jaw development

Question 1

what 2 mineralised regions is the skull made up of?

Answer 1

Neurocranium

• protective case for the brain

Viscerocranium

• Jaw skeleton

Question 2

why did the neurocranium form? in evolution

Answer 2

Neurocranium formed in response to expanding brains of fish ancestors

Question 3

what is the viscerocranium derived from - evolutionary?

Answer 3

Skeletal elements of viscerocranium derived from fish gills

• Branchial arches modified from primitive fish gill bars

Question 4

what is the neurocranium divided into?

Answer 4

Membranous neurocranium

• Consists of flat bones surrounding brain.

Cartilaginous neurocranium (Chondrocranium)

• Bones at base of skull.

Question 5

what are the differences in how membranous neurocranium and cartilaginous neurocranium are formed?

Answer 5

• Membranous neurocranium formed by intramembranous ossification.
• Cartilaginous neurocranium formed by endochondral ossification.

but both are formed by neural crest cells

Question 6

what can occur when formation and mineralisation of the neurocranium occurs?

Answer 6

Craniosynostosis (abnormal skull shape).

• When formation does not occur as it should

Premature closure & fusion = skull deformities.

• E.g. Aperts syndrome (coronal sutures).
• Cranium adopts a ‘tower skull’ to accommodate brain.

Question 7

what is the viscerocranium formed from? when?

Answer 7

Formed mainly from the first & second visceral (pharyngeal) arches (mesoderm origin).

• First arch gives rise to = Mandibular arch.
• Second = Hyoid arch.

Develop around week 4.

Question 8

what is the viscerocranium divided into?

Answer 8

membranous viscerocranium

chondral viscerocranium.

Question 9

what is the viscerocranium formed by?

Answer 9

migrating neural crest cells.

Question 10

what is the membranous viscerocranium divided into?

Answer 10

First visceral arch divided into 2 processes :

• Maxillary process (cranial part)
• Mandibular process (caudal part)

Question 11

what does the maxillary process give rise to?

Answer 11

Maxilla bone

Zygomatic bone

Temporal bone

Question 12

what does the mandibular process give rise to?

Answer 12

Mandibular bone

Question 13

what is an important structure in the formation of the mandible??

Answer 13

Meckel’s cartilage

Question 14

what is the role of meckel’s cartilage

Answer 14

Matrix template / framework = support

Support mineralisation process

but does not contribute to the mandible itself

Question 15

how is the mandible formed?

Answer 15

Meckel’s cartilage provides a template

Mesenchymal tissue surrounding Meckel’s cartilage undergoes ossification to form the mandible

• initially on lateral aspect

Meckel’s cartilage is gradually degraded and lost

Question 16

what occurs during week 6 of intra-uterine life in mandible development?

Answer 16

• meckel’s cartilage extends into hyaline cartilage rod
  
  • from otic capsule near developing ear to midline fusion
• 2/3 along the cartilage - mandibular division of CN V divides into ligual and inferior alveolar nerve
  
  • inferior alveolar nerve then divides into incisive and mental nerves
• Mesenchymal condensation occurs between incisive & mental nerves
  
  • Primary site of ossification

Question 17

where is the primary site of ossification?

Answer 17

between incisive & mental nerves

Question 18

what occurs during week 7 of intra-uterine life in mandible development?

Answer 18

intra-membranous ossification begins

• bone formation spreads to midline and posteriorly
  
  • spread along lateral aspect of Meckel’s cartilage forms trough of medial and lateral plates
  • these unite beneath incisive nerve
    
    • will form incisive canal
• this spreads to midline but fusion does not occur until after birth

Question 19

what occurs during week 10 of intra-uterine life in mandible development?

Answer 19

• rudimentary mandible is formed by intra-membranous ossification
• Meckel’s cartilage (B) forms malleus & spheno-mandibular ligament
• meckel’s cartilage resorbs

Question 20

what occurs during week 12 of intra-uterine life in mandible development?

Answer 20

• condylar cartilage appers
  
  • forms cone-shaped mass - occupies most of ramus
  • is rapidly converted to bone by endochondrak ossification

Question 21

what occurs during week 20 of intra-uterine life in mandible development?

Answer 21

• thin layer of cartilage remains at condylar head
• coronoid cartilage appears
  
  • fuses with ramus and disappears before birth
• symphysial cartilages appear
  
  • fuses with ramus and disappears during 1st year of life

Question 22

how does the maxilla develop?

Answer 22

Develops from centre (ventral) region of viscerocranial mesenchyme of the maxillary process of 1st arch

No arch cartilage or primary cartilage unlike in mandible

Question 23

what occurs during week 8 of intra-uterine life in maxilla development?

Answer 23

• cartilage of nasal capsule surrounded by MSC, which condense and ossify by intra-membranous ossification
• centre of ossification = where anterior superior alevolar nerve and infra-orbital nerve divide
• bone spreads :
  
  • posteriorly below orbit towards developing zygoma,
  • anteriorly towards incisor region
  • superiorly to form the frontal process.
• Ossification also spreads to palatine process, forming hard palate
• Medial alveolar plate develops from junction of palatal process & main body of maxilla & maxillary tooth germs become enclosed in bony crypts.
• 2° cartilage, the zygomatic (malar) cartilage appears in developing zygomatic process & helps development.

Question 24

what are the similarities in development of the mandible and maxilla?

Answer 24

Begin from single centres of ossification in close relation to the bifurcation of nerves.

Both develop an alveolar element, related to developing teeth.

Both develop 2° cartilages to assist in their growth, but no 1° cartilage in maxilla development.

Bulk of mandible & all of maxilla formed by intramembranous ossification.

• Only mandibular condyles formed by endochondral ossification.

Question 25

what is intra-membranous ossification?

Answer 25

E.g. body of mandible & maxilla

• Bone formation directly within the mesenchyme
• MSCs differentiation = osteoblasts.
• ECM synthesis / secretion (osteoid).
• Matrix-mediated mineralisation.
• Remodelling / turnover

Question 26

what is endochondral ossification?

Answer 26

E.g. mandibular condyle & syphysis, long bones

Hyaline cartilage formation.

• Chondrocytes undergo proliferation & hypertrophy.
• Cartilage ECM forms trabeculae & mineralises.
• Chondrocytes die.
• Bone marrow cells & blood vessels enter.
• Lining osteoblasts form lamellae & ECM = bone.
• Mineralised cartilage degraded.

Question 27

what is the structure of alveolar bone?

Answer 27

Alveolar process is part of jaw bone that contains tooth socket

Consists of outer cortical plate, central cancellous bone & bone lining tooth socket

Cortical plates & bone lining socket meet at alveolar crest

Bone lining socket is called bundle bone

Question 28

what are the properties of bundle bone?

Answer 28

• Immature bone (similar to woven bone).
• Bone lining tooth socket / lamina dura.
• Perforated by nerves & blood vessels (cribriform plate).
• Histologically consists of bundles of collagen fibres, arranged in lamellae running parallel to socket.
• Embedded with extrinsic collagen fibres from PDL (Sharpey’s fibres, SF).