Development of the Face & Palate

Question 1

When do the pharyngeal arches begin to form?

How do they develop over the coming weeks to have a characteristic appearance?

Answer 1

• pharyngeal arches appear in the 4^th and 5^th weeks of development
• they initially consist of bars of mesenchymal tissue separated by deep clefts (pharyngeal clefts)
• with development of numerous arches and clefts, a number of outpocketings called pharyngeal pouches begin to appear along the lateral walls of the pharynx

Question 2

What is significant about the contents of each pharyngeal arch?

Answer 2

• each pharyngeal arch is characterised by its own muscular components
• the muscular components of each arch have their own cranial nerve, and they carry this nerve component wherever they migrate
• each arch also has its own arterial component

Question 3

How do the pharyngeal arches contribute to formation of the face at the end of the 4^th week?

What 5 mesenchymal prominences can be recognised by day 42?

Answer 3

End of week 4:

• the centre of the face is formed by the stomodeum surrounded by the first pair of pharyngeal arches

By day 42:

• there are 5 mesenchymal prominences now visible:

1. mandibular prominences (1^st pharyngeal arch) caudal to the stomodeum
2. maxillary prominences (dorsal portion of 1^st pharyngeal arch) lateral to the stomodeum
3. frontonasal prominence that is a rounded elevation cranial to the stomodeum

Question 4

What is the composition of each pharyngeal arch?

Answer 4

• each pharyngeal arch consists of a core of mesenchyme
• this core consists of mesenchyme derived from paraxial and lateral plate mesoderm, as well as substantial numbers of neural crest cells (NCCs)
• NCCs migrate into the arches to contribute to skeletal components of the face
• each arch is covered on the outside by surface ectoderm** and on the inside by **epithelium of endodermal origin

Question 5

What is formed by the mesenchymal core and NCCs of each pharyngeal arch?

Answer 5

• the original mesoderm of the arches gives rise to the musculature of the face and neck
• the NCCs give rise to viscerocranium (facial skeleton) of the face

Question 6

How many pharyngeal arches are present?

Answer 6

• the pharyngeal arches are 1, 2, 3, 4 and 6
• the 5^th pharyngeal arch is rudimental and transient and quickly regresses so it can no longer be seen

Question 8

What muscles arise from the first pharyngeal arch?

What is their nerve supply?

Answer 8

• muscles of mastication (temporalis, masseter, pterygoids)
• anterior belly of digastric
• mylohyoid
• tensor tympani
• tensor palatini
• all of these muscles are supplied by the mandibular branch of the trigeminal nerve

Question 9

What group of muscles form from the second pharyngeal arch?

What is their nerve supply?

Answer 9

• muscles of facial expression
• the facial nerve supplies all the muscles derived from the second arch

Question 10

How many pharyngeal clefts are present in the 5-week embryo?

Which structures are formed from these clefts and how do they develop?

Answer 10

• there are 4 pharyngeal clefts in the 5-week embryo
• only the first pharyngeal cleft goes on to form any adult structures - the external acoustic meatus
• the second pharyngeal cleft ectoderm grows caudally to cover the rest of the clefts

Question 11

What are pharyngeal pouches and how many are present?

When does formation of these pouches occur?

Answer 11

• there are 4 pairs of pharyngeal pouches that form prior to NCC migration
• they are epithelial endodermal structures that form on the inside of the pharynx, between the arches
• they are formed by migration of endodermal cells laterally**, which is stimulated by **FGFs

Question 12

What is signalling from pouch endoderm important for?

Answer 12

• signalling from pouch endoderm is important in the regulation of skeletal patterning
• it was previously thought that NCCs within the core of the arches were responsible for this as they form the skeletal components
• Formation of pharyngeal pouches occurs prior to NCC migration and even takes place in the absence of crest cells

Question 13

Where do the NCCs that migrate into the core of the pharyngeal arches originate from?

What transition must they undergo and what factors are important in ensuring this happens?

Answer 13

• NCCs originate from neuroepithelial cells adjacent to the surface ectoderm all along the edges of the neural folds
• BMP signalling is important in establishing this edge region
• BMP signalling then regulates WNT expression** to cause NCCs to undergo an **epithelial-to-mesenchymal transition
• this allows them to begin their migration into the surrounding mesenchyme

Question 15

How are pharyngeal pouches formed?

What factor is important in this process?

Answer 15

• pharyngeal pouches are formed by migration of endoderm cells laterally
• this migration is stimulated by fibroblast growth factors (FGFs)

Question 16

What expression of genes is exhibited by pharyngeal pouches as they form?

Answer 16

• BMP7** is expressed in the **posterior endoderm of each pouch
• FGF8 is expressed in the anterior endoderm of each pouch
• PAX1 is restricted to the dorsalmost endoderm of each pouch
• SHH is expressed in the posterior endoderm of the 2^nd and 3^rd pouches

Question 17

What is the significance of the characterisitc pattern of gene expression of the pharyngeal arches?

What is this process dependent upon?

Answer 17

• the highly specific expression of genes within each pharyngeal pouch regulates differentiation and patterning of pharyngeal arch mesenchyme into specific skeletal structures
• this process is dependent on the mesenchyme, so is an example of an epithelial-mesenchymal interaction

Question 18

What transcription factors are present in the mesenchymal core?

How is this different between the arches?

Answer 18

• transcription factors within the arches include HOX genes and others that are carried there by the NCCs
• NCCs acquire their specific gene expression from the primitive brain regions that they originate from
• Different arches have different patterns of HOX gene expression within the mesenchyme
• NCCs travelling to the arches have originated from different primitive brain regions

Question 19

What is the epithelial-mesenchymal interaction involved in skeletal patterning?

Answer 19

• the pouch endoderm signals to the mesenchymal core to begin to express transcription factors (incl. HOX genes)
• as HOX gene expression is different in each arch, the interaction between the mesenchyme and the endoderm is different in each arch
• different expression patterns of transcription factors allow each arch to respond differently to the signals emanating from pouch endoderm
• this ultimately gives rise to different structures from each arch

Question 20

What is formed by the neural crest and mesenchyme within the frontonasal prominence?

What factors are important in this process?

Answer 20

• the upper facial skeleton is derived from NCCs that migrate into the frontonasal prominence
• signals emanating from the surface ectoderm and underlying areas of neuroepithelium dictate the fate of the mesenchyme
• SHH & FGF8 have major roles in patterning this area, but specific genetic interactions are not known

Question 21

Which of the 3 facial prominences are formed by the 1st pharyngeal arch?

Answer 21

• both the maxillary and mandibular prominences are formed by the 1st pharyngeal arch
• the frontonasal prominence is formed by proliferation of mesenchyme located ventral to the developing brain vesicles

Question 22

How can the 3 facial prominences appearing in the 4^th week be distinguished from each other?

Answer 22

• the maxillary prominences are located lateral to the stomodeum
• the mandibular prominences are located caudal to the stomodeum
• the frontonasal prominence constitutes the upper border of the stomodeum

Question 23

What are the 5 facial prominences mainly derived from?

Answer 23

• the mesenchyme of the facial prominences is primarily derived from NCCs
• this will go on to form bone, facial cartilage and ligaments
• the mesenchymal core proliferates and rapidly increases in size
• as the maxillary and mandibular processes increase in size, they begin to push other structures together

Question 24

What do local thickenings near the frontonasal prominence go on to form?

Answer 24

• on both sides of the frontonasal prominence, local thickenings of surface ectoderm form the nasal placodes
• during week 5, the nasal placodes invaginate to form nasal pits
• this creates a ridge of tissue that surrounds each pit and forms the nasal prominences
• the prominences on the outer edge of the pits are the lateral nasal prominences
• the prominences on the inner edge of the pits are the medial nasal prominences

Question 25

Following the formation of the medial and lateral nasal prominences, what happens to the maxillary prominences?

Answer 25

* over the next 2 weeks, the maxillary prominences continue to **increase in size** * they **simultaneously grow medially** to **push the _medial nasal prominences_ toward the _midline_** * the cleft between the medial nasal prominence and the maxillary prominence on each side is lost and they **fuse together** * the **_upper lip_** is formed from the **_2 medial nasal prominences_** and the **_2 maxillary prominences_** * *the lateral nasal prominences do not participate in formation of the upper lip*

Question 26

How is the nasolacrimal duct formed?

Answer 26

* initially, the **maxillary prominences** and the **lateral nasal prominences** are separated by a deep furrow - the **_nasolacrimal groove_** * **ectoderm** in the **floor of the nasolacrimal groove** forms a **_solid epithelial cord_** that **detaches** from the overlying ectoderm * the cord **canalises** to form the **_nasolacrimal duct_** * its **upper end widens** to form the **_lacrimal sac_**

Question 27

What happens following detachment of the nasolacrimal duct? Where does this duct run to and from?

Answer 27

* the **maxillary** and **lateral nasal prominences** **_merge with each other_** after detachment of the nasolacrimal duct * the duct then runs from the **medial corner of the eye** to the **inferior meatus** of the nasal cavity * the maxillary prominences **enlarge** to form the **_cheeks**_ and _**maxillae_**

Question 28

What forms the intermaxillary segment? What is it continuous with?

Answer 28

* as a result of medial growth of the maxillary prominences, the **2 medial nasal prominences merge _not only at the surface_** but at a **_deeper level_** * the structure formed by the **_2 merged medial nasal prominences_** is the intermaxillary segment * the intermaxillary segment is continuous with the **rostral portion of the nasal septum** (formed by the frontal prominence)

Question 29

What are the 3 parts of the intermaxillary segment?

Answer 29

**_Labial part:_** * forms the **philtrum** of the upper lip **_Upper jaw:_** * gives rise to the **4 incisor teeth** **_Palatal part:_** * gives rise to the triangular **primary palate**

Question 30

How is the secondary palate formed and what is it formed from? When does this formation occur?

Answer 30

* the definitive palate is formed from **2 shelf-like outgrowths** from the **maxillary prominences** - the **_palatine shelves_** * the palatine shelves appear in the **6^th week** and are directed **obliquely downwards** on **either side of the tongue** * in **week 7**, the **_palatine shelves ascend_** to attain a **_horizontal position above the tongue_** * the 2 palatine shelves **_fuse_** to form the **secondary palate**

Question 31

How is the definitive palate formed? What delineates the primary and secondary palates?

Answer 31

* the definitive palate is formed when the **_secondary palate_** (from palatine shelves) **_fuses with the primary palate_** (from intermaxillary segment) * the **_incisive foramen_** delineates where the primary palate fuses with the secondary palate * at the same time as this fusion, the **nasal septum grows down** to join with the **cephalic aspect** of the newly formed palate

Question 32

Why does abnormal facial development usually occur? What usually occurs alongside this?

Answer 32

* it occurs as a result of **_disruption of neural crest cells_** * NCCs are essential for formation of **much of the craniofacial region**, so their disruption results in craniofacial abnormalities * NCCs also contribute to the **_conotruncal endocardial cushions_**, which **septate the outflow tract** of the heart into pulmonary and aortic channels * this means infants with craniofacial abnormalities often also have **_cardiac abnormalities_** * *e.g. tetralogy of Fallot, persistent truncus arteriosus, transposition of the great vessels*

Question 34

What is the triad associated with Robin sequence? What type of syndrome is this?

Answer 34

1. **micrognathia** (small oral opening) 2. **cleft palate** 3. **glossoptosis** (posteriorly placed tongue) * it is a form of ***"first arch syndrome"*** that most severely affects the **_mandible_** * poor growth of the mandible results in a posteriorly placed tongue that **fails to drop from between the palatal shelves**, preventing their fusion

Question 35

What can cause Robin sequence?

Answer 35

* it can occur independently or in association with other syndromes * it can be due to genetic or environmental factors * it can also be caused by a deformation, such as **oligohydramnios**, in which the **chin may be compressed against the chest**

Question 36

What causes Treacher-Collins syndrome (mandibulofacial dystosis)?

Answer 36

* it is a rare **autosomal dominant** condition * *60% of cases arise as new mutations* * it is caused by a **mutation in the _TCOF1 gene_** * the product of the TCOF1 gene is a protein called **_treacle_**, which **_prevents apoptosis_** and **_maintains proliferation in NCCs_** * treacle is **_not_** responsible for **regulating migration** of NCCs as this still happens normally

Question 37

Why do the roles of the treacle protein reflect the presentation of mandibulofacial dystosis?

Answer 37

* treacle is needed to **_prevent apoptosis_** and **_maintain proliferation_** of NCCs * the NCCs **still migrate** to the relevant areas, but they do not differentiate properly and undergo apoptosis before they can develop * this results in **_insufficient tissue_** being present to **develop all of the relevant facial structures**

Question 38

What possible environmental factor could be associated with Treacher-Collins syndrome?

Answer 38

* the majority of cases are genetic * teratogenic doses of **_retinoic acid_** may also be responsible

Question 39

What are the main characteristic features of Treacher-Collins syndrome?

Answer 39

* it is characterised by **hypoplasia of the maxilla, mandible and zygomatic arches** (which may be absent) * the TMJ is badly affected and is positioned in the wrong location

Question 40

What are other presentations that are commonly seen in Treacher-Collins syndrome?

Answer 40

* **_cleft palate_** is common (as well as other features of Robin sequence) * **external ear defects** accompanied by **atresia of the auditory canals** and abnormalities of the **middle ear ossicles** * *bilateral conductive hearing loss is often present as a result* * eyes are usually involved in **_down-slanting palpebral fissures_** and **_lower eyelid colobomas_**

Question 41

What causes DiGeorge syndrome?

Answer 41

* it is a common deletion syndrome caused by a **_microdeletion_** in the **q11.2** region of **chromosome 22** (**_22q11.2 deletion syndrome_**) * it occurs due to a **genetic malformation of the NCCs**, which contribute to the mesenchyme * the mesenchyme forms the **connective tissue of glands** and interacts with **endodermal cells** * *these endodermal cells form the cells of the thymus and parathyroid glands*

Question 42

What is the triad of symptoms associated with DiGeorge syndrome?

Answer 42

1. shortened philtrum 2. low set ears 3. nasal clefts

Question 43

What are some other associations of DiGeorge syndrome?

Answer 43

1. absent thymus & parathyroid glands 2. defective T-cell function 3. cardiac abnormalities 4. facial dysmorphology (tends to be mild - smaller mouth) 5. learning disabilities

Question 44

Why are an absent thymus & parathyroid glands seen in DiGeorge syndrome? What are the consequences?

Answer 44

* these fail to develop from the **3rd and 4th pharyngeal pouches** * this leads to **_congenital hypoparathyroidism_**, which presents as **seizures** * it is caused by **_disruptive signalling_** between the **pharyngeal endoderm** and the **NCCs**

Question 45

Why does defective T-cell function occur in DiGeorge syndrome? What is this associated with?

Answer 45

* **thymic hypoplasia / aplasia** disrupts the immune system's T-cell mediated response * the thymus gland is important for the immune response in children * this presents with **frequent infections**

Question 46

Why might there be abnormal midline craniofacial development? What signalling factor may be implicated?

Answer 46

* this occurs when there is **_insufficient midline tissue_** early on in development (during **gastrulation**) * this results in **underdevelopment of the forebrain** and **frontonasal prominence** * ***_SHH_*** is important for development of midline structures

Question 47

What are the 3 most common causes of midline disorders?

Answer 47

1. SHH mutations 2. maternal diabetes 3. alcohol * there is not enough tissue present in the midline to form all of the structures that should be present

Question 48

What are the characteristics of the most severe midline disorder (holoprosencephaly)?

Answer 48

* **medial nasal prominences do not form** due to insufficient tissue * the lateral nasal prominences fuse to form a proboscis with a single midline opening * **narrow head** * **fusion of the eyes (**synophthalmia) * the **upper lip** is formed by the fusion of the **maxillary prominences** * there is a **single midline brain ventricle** formed by the fusion of the lateral ventricles

Question 49

What are the 3 types of orofacial clefts?

Answer 49

1. **lip only (CLO)** 2. **lip + palate (CLP)** 3. **palate only** * all of these can be unilateral or bilateral * midline and oblique clefts can occur, but these are much less common

Question 50

How can orofacial clefts be divided into anterior and posterior deformities?

Answer 50

* the **_incisive foramen_** is the dividing landmark between anterior and posterior deformities * the incisive foramen is the **midline landmark** between the **primary** and **secondary palates**

Question 51

What are examples of an anterior cleft deformity? What is the underlying cause of this?

Answer 51

* anterior deformities are those occurring **_anterior to the incisive foramen_**, such as: 1. cleft lip (unilateral or bilateral) 2. cleft jaw 3. cleft between primary and secondary palates * these defects occur due to **partial or complete lack of fusion** of the **maxillary prominence with the medial nasal prominence** on one or both sides

Question 52

What are examples of posterior cleft deformities? What is the underlying cause?

Answer 52

* posterior clefts are those that lie **_posterior to the incisive foramen_**, such as 1. cleft uvula 2. cleft secondary palate * these result from **_failed fusion of the palatine shelves_**, which may be due to: 1. shelves are **too small** 2. shelves **fail to elevate** 3. **inhibition of the fusion** process itself 4. **failure of the tongue to drop between the shelves** due to micrognathia

Question 53

What is the third type of cleft that can occur?

Answer 53

* it is possible to have a combination of **both anterior and posterior** clefts

Question 54

What is the underlying process that results in unilateral and bilateral cleft lips? How is it treated?

Answer 54

**_Unilateral cleft lip:_** * occurs when the **maxillary prominence** **_fails to fuse_** with the **merged medial nasal prominences** **_Bilateral cleft lip:_** * occurs when there is **failure of the _BOTH_ maxillary prominences** to fuse with the merged medial nasal prominences * treatment for cleft lip is performed by **3 months** and tends to have positive results

Question 55

What are the 4 main causes of cleft lip +/- palate? What are the ethnic differences in the presentation of this condition?

Answer 55

1. syndromic (e.g. Van der Woude syndrome) 2. related to genetics but not syndromic 3. non-syndromic 4. teratogen exposure - smoking, anti-epileptics, benzos & rubella * has a male predominance and incidence is higher in Asians & Native Americans

Question 56

What is Van der Woude syndrome and why does it occur?

Answer 56

* it is the most common syndrome that is associated with **cleft lip +/- palate** * it is an **autosomal dominant disorder** that occurs due to mutations in ***_INTERFERON REGULATORY FACTOR 6_*** (IRF6) * IRF6 is expressed in the **_medial (fusing) edge_ of the palatal shelves**

Question 57

Why do oblique facial clefts occur?

Answer 57

* due to failure of the **maxillary prominence** to merge with its corresponding **lateral nasal prominence** along the **line of the nasolacrimal duct** * the nasolacrimal duct is usually exposed to the surface

Question 58

What are the problems associated with cleft lip and palate?

Answer 58

1. **difficulty feeding** can lead to **poor weight gain / failure to thrive** 2. **aspiration pneumonia** 3. hearing problems 4. speech problems 5. dental problems (incisor teeth cannot form if intermaxillary segment is affected) 6. cosmesis