3. Skull Development/Organization

Question 1

Which bones develop from endochondral ossification?

Answer 1

Hyaline cartilage replaced by bone
Neurocranium: Skull Base - Ethmoid, Petrous (Temporal), Base of Occipital, Sphenoid
Viscerocranium: Little Neck Bones - Inf Concha, Hyoid, Part of Mandible, Thyroid Cartilage, Ear Ossicles (Incus, Stapes, Malleus)

Question 2

Which bones develop from intramembranous ossification?

Answer 2

Direct sheets of mesenchymal c.t. into bone (no cartilage)
Neurocranium: Frontal, Parietals, parts of temporal/occipital/ethmoid/sphenoid
Viscerocranium: Lacrimals, Zygomas, Nasals, Vomer, Palatines, Maxilla, Part of Mandible

Question 3

Which bones develop from head ectomesenchyme?

Answer 3

Anterior Skull and middle ear

ALL facial bones, frontal, sphenoid (anterior part), Squamous (Temporal)

Question 4

Which bones develop from paraxial mesoderm?

Answer 4

Posterior Skull

Parietals, Occipitals, Petrous (Temporal), Sphenoid (posterior)

Question 5

What is a structure near the line splitting bones of ectomesenchyme and mesoderm derived bone?

Answer 5

Sella turcica of sphenoid (pituitary fossa)

Question 6

Achondroplasia - what causes it, how it affects bones

Answer 6

mutant FGFR3 = decreased cartilage matrix production = decreased endochondral bone ossification
Prominent forehead, shortened midface, depressed nasal bridge, shortened skull base
Only affects CHONDROCRANIUM

Question 7

Craniosynostosis - what is it, secondary problems

Answer 7

Failure of proper mLc signalling at primordia boundaries = differential shutdown of growth
Premature suture fusion of primordia boundaries - bones continue to grow in different directions
most common developmental disorder of skull vault formation
Secondary problems = high intracranial pressure, facial deformities, breathing difficulty

Question 8

Chordoma - what is it, where is it located

Answer 8

rare tumor of skull base
skeletal tissue neoplasm from persistent portions of notochord
most commonly located in midline basi-sphenoid = distal end of notochord where paraxial mesoderm meets head mesenchyme!