SM_03b: Development & Organization of the Skull

Question 1

Which facial bones are singular?

Answer 1

Singular facial bones

• Occipital
• Frontal
• Sphenoid
• Mandible
• ethmoid
• Vomer

Question 2

Which facial bones are double?

Answer 2

Double facial bones

• Nasal
• Lacrimal
• Maxilla
• Parietal
• Temporal
• Zygoma

Question 3

____ is the bones providing protective housing of the brain and consists of the ____ and ____

Answer 3

Neurocranium is the bones providing protective housing of the brain and consists of the calvarium and basicranium

• Calvarium: parietals, frontal, occipital (squamous), temporal (squamous)
• Basicranium: supportive floor - ethmoid, sphenoid (primary components), temporal (petrous and mastoid), occipital (basilar portion with condyles)

Question 4

____ is the facial skeleton and is derived from ____

Answer 4

Viscerocranium is the facial skeleton and is derived from branchial arches

Question 5

____ is the major region of the adult skull that is particularly large in humans

Answer 5

Neurocranium is the major region of the adult skull that is particularly large in humans

Question 6

Skull structure is comprised of ___

Answer 6

Skull structure is comprised of a series of distinct but interconnected bony cavities which house and protect soft tissues associated with special senses and respiration

• Cranial cavity: brain, etc
• Orbital cavity: eyeball, etc
• Temporal bone cavities: hearing, balance
• Nasal cavity: olfaction, respiration
• Oral cavity: taste, chewing, etc

Question 7

____ and ____ may follow connections between cavities

Answer 7

Infectious agents and neoplasms may follow connections between cavities

Question 8

Endochondral ossification is when ___ and occurs in the ___ and ___

Answer 8

Endochondral ossification is when a hyaline cartilage model is replaced by bone and occurs in postcranial bones and skull base

make up skullbase (chondrocranium) plus the little bones of the middle ear and neck

Question 9

Intramembranous ossification is when ___ and occurs in ___ and ___

Answer 9

Intramembranous ossification is when direct sheets of mesenchymal connective tissue transform into bone and occurs in large portions of the skull and most of the clavicle

Question 10

Each of the major divisions of the skull have bones formed via ____ and ____

Answer 10

Each of the major divisions of the skull have bones formed via endochondral and intramembranous ossification

Question 11

This is the ____ bone

Answer 11

This is the sphenoid bone

Question 12

Achondroplasia affects bones of the ____ in the skull

Answer 12

Achondroplasia affects bones of the chondrocranium in the skull

(achondroplasia is a disease of defective endochondral ossification)

Question 13

Summarize the skull ossification pathways

Answer 13

Skull ossification pathways

• Endochondral ossification: bones along the midline cranial base plus the middle ear and hyoid bones
• Intramembranous ossification: bones of the calvarium portion of the neurocranium, surrounding the sides and top of the brain, plus most of the bones of the face

Question 14

Bones of the skull develop from either the ___ or from ___

Answer 14

Bones of the skull develop from either the neural crest mesenchyme originating around the developing neural tube (brain) or from paraxial mesoderm along the craniad (rostral) end of the developing notochord and vertebral column

(pathways of ossification do not correspond to tissue primordia of the skull bones, ectomesenchyme derived from neural crest)

Question 15

Neural crest ____ gives rise to many bones in the ____ portion of the skull

Answer 15

Neural crest ectomesenchyme gives rise to many bones in the anterior portion of the skull

(during weeks 4-6: connective tissue giving rise to most of the bones of the face and middle ear develops from ectomesenchyme which migrates from the neural crest into the first two arches)

Question 16

Bones shown in blue develop from ____, while those shown in red develop from ____

Answer 16

Bones shown in blue develop from neural crest ectomesenchyme, while those shown in red develop from paraxial mesoderm

(if anterior to a line through the sella turcica of the body of the sphenoid bone then arise from the ectomesenchyme)

Question 17

____ is premature fusion of one or more cranial suture and serves as the most common developmental of the calvarium (skull vault)

Answer 17

Craniosynostosis is premature fusion of one or more cranial suture and serves as the most common developmental of the calvarium (skull vault)

Question 18

____ and ____ are boundaries between mesenchyme derived from the neural crest and that derived from the paraxial mesoderm

Answer 18

Coronal and sagittal suture boundaries are boundaries between mesenchyme derived from the neural crest and that derived from the paraxial mesoderm

Question 19

Chordoma is a ____ that most commonly occurs in the ____

Answer 19

Chordoma is a rare neoplasm of skeletal tissues in adults derived from persistent portions of the notochord that most commonly occurs in the midline basisphenoid

(postsphenoid derives from cephalic mesoderm, more anterior portions of the midline skullbase are neural crest ectomesenchyme)

Question 20

Describe the clinical conditions that can remind you that different subregions of the skull have divergent developmental origins

Answer 20

Clinical conditions that can remind you that different subregions of the skull have divergent developmental origins

• Skullbase chordomas: divide between older paraxial mesoderm derived skull bones (or parts of bones such as the posterior sphenoid, where the notochord starts) and newer ectomesenchyme originating in the neural crest (anterior sphenoid)
• Craniosynostosis (premature fusion of the sagittal and coronal sutures): these sutures are boundary zones of paraxial mesoderm-derived skull bones and those derived from ectomesenchyme originating in the neural crest