development of skull

Question 1

Describe the Structures involved in the formation of skull

Answer 1

• Somites-At 5 weeks, roughly 44 pairs of somites can be seen
  ○ 4 occipital pairs
  ○ 8 cervical pairs
  ○ 12 thoracic pairs
  ○ 5 lumbar pairs
  ○ 5 sacral pairs
  ○ 3-5 coccygeal pairs
  
  • Six pairs of well-defined pharyngeal arches numbered 1, 2, 3, 4, 6
  • Mesenchymal core (paraxial mesoderm and neural crest cells)

Question 2

Describe the Neurocranium and viscerocranium

Answer 2

• Neurocranium (calvaria and basicranium) - 8 bones: 4 singular bones (Frontal, ethmoidal, sphenoidal, and occipital) 2 sets of bones bilateral pairs (Temporal and parietal)
  Viscerocranium (facial skeleton) -hangs down from the front of the neurocranium – mandible, vomer, maxillae, inferior nasal conchae, zygomatic, palatine, nasal, and lacrimal bones

Question 3

Describe the two major types of ossification

Answer 3

• Endochondral ossification-mesenchymal cells that differentiate into chondroblast. Chondroblast secrete extracellular matrix. Chondroblasts become encased in the cartilage matrix and become chondrocytes. The chondrocytes in the centre of the cartilage model increase in size, and some burst – releasing cell contents that trigger calcification. Chondrocyte cell death within the calcifying matrix forms small cavities for osteoblasts to move into. (chondrocytes are converted into osteoblast?)
  
  • Intermembranous ossification - forms flat bones. mesenchymal cells that differentiate into osteoblast. osteoblasts synthesise and secrete the osteoid, which is the unmineralized, organic portion of the bone matrix. As this secreted matrix becomes calcified with the binding of calcium, the matrix hardens and entraps the osteoblasts o they become osteoclast

Question 4

Which bones in the skull are formed by Endochondral ossification.

Answer 4

• Part of the occipital bone
  
  • Nasal capsule
  • All bones other than those few from the Intermembranous ossification
    ○ Best way to think is to remember what is developed by Intermembranous ossification and realise its everything but those (with occipital bone exceptions)

Question 5

Which bones in the skull are formed by Intermembranous ossification.

Answer 5

• Part of the occipital bone
  
  • Frontal bone
  • Parietal bone
  • Maxilla
  • Mandible
  • Squamous part of temporal bone
  • Zygomatic bone

Question 6

Which structures are from crest cells and chich are from paraxial mesoderm cells?

Answer 6

• The bones of neural crest origin (blue) include:
  ○ ethmoid bones
  ○ greater and lesser wings of the sphenoid bone
  ○ anterior part of the sella turcica
  ○ Frontal bone
  ○ Everything anterior to the sella turcica
  
  • The bones of paraxial mesoderm origin(pink) include:
    ○ posterior part of the sella turcica
    ○ body of the sphenoid
    ○ base of the occipital bone
    ○ petrous part of the temporal bone
    ○ upper (interparietal) part of the occipital bone
    ○ parietal bones
    ○ Everything posterior to the sella turcica

Question 7

What is the role of fontanelles?

Answer 7

• Six fontanelles—are present where several sutures meet.
  
  • the softness of the bones and their loose connections at the sutures enable the calvaria
  • to undergo changes of shape during birth, called moldin. Also allows the brain to grow with the skull to accommodate for it
  • The frontal bones become flat, the occipital bone is drawn out, and one parietal bone
  • slightly overrides the other one during birth for easier delivery
  • Within a few days after birth, the shape of the calvaria returns to normal
  • Ossification happens at officiation sites causing the bone to push outward/grow and eventually close the fontanelles

Question 8

Describe the Outline of pharyngeal arches

Answer 8

• 1st arch: Derived from arch cartilages (originating from neural crest cells): from maxillary cartilage: incus
  
  • From Meckel’s cartilage: malleus Mandible (originate from neural crest cells)
  • 2nd arch: Stapes, styloid process, lesser horns and part of the body of hyoid(originate from neural crest cells)
  • 3rd arch: Lower rim and part of the body of hyoid (originate from neural crest)
  • 4th arch: Thyroid and epiglottal laryngeal cartilages (derived from neural crest cells and mesoderm)
  • 6th arch: Remaining laryngeal cartilages (derived from mesoderm)

Question 9

Describe the development of the mandible

Answer 9

• Intramembranous ossification
  
  • Coronoid region and condyle are formed by endochondral ossification - 10–14weeks.
  • Mental ossicles – 7 months.
  • This complex system enables the mandible to grow at both proximal and distal ends, analogous to a long bone the first structure to develop in the primodium of the lower jaw is the mandibular division of trigeminal nerve that precedes the mesenchymal condensation forming the first arch
  • Some mesenchymal cells enlarges and form osteoblasts – ossification centre – at the division of IAN. growth of mandible
  • one or two cartilage appear and ossify in 7th month of inter-uterine life to become mental ossicles
  • At the end of first year symphysial cartilage is replaced by bone
  • Mandible moves and grows downward & forward.
  • Appositional growth occurs on posterior border of the ramus and on the alveolar process.
  • Resorption occurs along the anterior border of ramus growth of mandible

Question 10

Describe the Clinical correlation associated

Answer 10

• related to aberrant signalling at the boundary between neural crest and paraxial mesoderm parts of the skull.
  
  • The intersection of the coronal and sagittal sutures is at one such junction. The frontal bone is of neural crest origin and the two parietal bones are from paraxial mesoderm.
  • Cells from these two sources mingle and interact for precise timing of suture closure.