L1 Face and Skull Development

Question 1

What are the facial processes and when do they appear embyonically?

Answer 1

The 5 facial processes appear by the end of week 5 around the stomodeum, they are produced by proliferating zones of mesenchyme lying beneath the surface ectoderm:

• Frontonasal process
• 2 maxillary processes
• 2 mandibular processes

Question 2

Describe the frontonasal process.

Answer 2

Formed by a mesenchymal proliferation ventral to the forebrain.
- Forms the forehead, dorsum and apex of nose

Question 3

Describe the maxillary processes.

Answer 3

Derived from the first branchial arch, forms from the sides of the stomodeum.
- Form the upper cheeks, majority of maxilla, secondary(definite) palate and upper lip

Question 4

Describe the mandibular processes.

Answer 4

Derived from the first branchial arch, formed by the caudal boundary of the stomodeum.
- Form the lower cheeks, mandible and lower lip

Question 5

Describe the development of the nose by week 5 i.u.

Answer 5

• Localised thickenings of ectoderm give rise to the nasal placodes
• These placodes enlarge and invaginate into the underlying mesenchyme to form nasal pits
• There is a ‘raised horseshoe rim’ of placodes which divide into the lateral and medial nasal processes

Question 6

Describe the development of the nose at weeks 6-7.

Answer 6

• Medial nasal processes migrate inwards to eachother and fuse, forming a primordia of the nasal bridge and septum
• The maxillary processes merge with the lateral nasal processes to begin forming the sides of the nose

Question 7

Describe the development of the intermaxillary segment.

Answer 7

At weeks 6-7:

• The inferior tips of the medial nasal processes expand laterally and inferiorly, fusing to form the intermaxillary segment
• Tips of maxillary processes grow to meet the intermaxillary segement and fuse

The intermaxillary segment gives rise to the philtrum, premaxilla (site of upper incisor development) and the primary palate

Question 8

What are the 3 facial placodes?

Answer 8

• Nasal placodes (nasal pits, nose)
• Lens placodes (lens of the eye)
• Otic placodes (inner ear)

Question 9

Describe the formation of the nasal passages.

Answer 9

• The nasal pits deepen as the facial processes grow
• Behinds the openings, the pits fuse to form an enalrged nasal sac
• Floor of nasal sac proliferates and forms a thickened plate seperating the sac from the oral cavity, the plate is called the nasal fin
• Nasal fin thins due to developing vacuoles and forms the oronasal membrane

Question 10

Describe the formation of the nasolacrimal duct.

Answer 10

Week 6:

• There is a cleft between the lateral nasal process and adjacent maxillary process called the nasolacrimal groove
• Ectoderm of the floor of the groove thickens to form a solid epithelial cord
• Cord sinks into mesenchyme forming the nasolacrimal duct
• At the cranial end the duct expands to form the lacrimal sac

Question 11

If the nasolateral processes fail to fuse properly with the maxillary processes what pathology occurs?

Answer 11

Cleft lip and palate.

Can be detected in ultrasounds and MRI scans.

Question 12

What does the skull develop from?

Answer 12

Develops from mesenchyme around the developing brain.

Question 13

What are the 2 divisions of the skull?

Answer 13

Neurocranium: protective case for the brain (divided into chondrocranium and membranous neurocranium)
Viscerocranium: jaw skeleton

Question 14

Describe the structure of the chondrocranium.

Answer 14

• Composed of 3 cartilaginous proccesses (parachordal, hypophyseal, prechordal)
• The 3 cartilages are derived from neural crest cells and ossify via endochondral ossification

Question 15

Describe the parachordal cartilage.

Answer 15

• Most dorsal
• Forms at cranial end of notochord
• Forms boundaries of foramen magnum and the base of the occipital bone

Question 16

Describe the hypophyseal cartilages.

Answer 16

• 2 cartilages
• Form around the developing pituitary gland
• Ultimately fuse to form the sella turcica and body of the sphenoid bone

Question 17

Describe the prechordial cartilages.

Answer 17

• Most ventral
• 2 cartilages
• Ultimately fuse to from the body of the ethmoid bone and contributes to the nasal septum

Question 18

What are sensory capsules?

Answer 18

Capsules derived from paraxial mesoderm and neural crest cells.

Question 19

Describe the nasal capsule.

Answer 19

• Develops around nasal sacs

- Ultimately forms the nostril cartilages, conchae and parts of the ethmoid bone

Question 20

Describe the otic capsules.

Answer 20

• Develops around the otic/auditory vesicle
• Ultimately fuses with parts of the parachordial cartilages to form periotic capsules
• Periotic capsules later ossify as petrous and mastoid parts of the temporal bones

Question 21

What is the neurocranium divided into?

Answer 21

The chondrocranium: forms bones of the base of the skull

The membranous neurocranium: flat bones

Question 22

How does the membranous neurocranium ossify?

Answer 22

Intra-membranous ossification (by sites called bone spicules in the mesenchyme covering the brain).

Ossification sites develop into flat bones of the calvaria.

Question 23

What are the bones of the calvaria?

Answer 23

• Frontal
• Parietal
• Squamous temporal
• Squamous occipital

Question 24

When do the bones of the calvaria fuse?

Answer 24

The bones of the calvaria are seperated by unfused soft fibrous sutures.
These fuse during early childhood and even into adulthood.

Question 25

What are fontanelles?

Answer 25

Fontanelles are the soft spots on an infant’s head where the bones of the calvaria have not yet fused.
Can feel their heartbeat here, it will swell if the child has a viral infection.