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Embryology > Development of the Face & Palate > Flashcards

Development of the Face & Palate Flashcards

1
Q

When do the pharyngeal arches begin to form?

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

A
  • pharyngeal arches appear in the 4th and 5th 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
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2
Q

What is significant about the contents of each pharyngeal arch?

A
  • 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
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3
Q

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

What 5 mesenchymal prominences can be recognised by day 42?

A

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 (1st pharyngeal arch) caudal to the stomodeum
  2. maxillary prominences (dorsal portion of 1st pharyngeal arch) lateral to the stomodeum
  3. frontonasal prominence that is a rounded elevation cranial to the stomodeum
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4
Q

What is the composition of each pharyngeal arch?

A
  • 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
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5
Q

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

A
  • 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
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6
Q

How many pharyngeal arches are present?

A
  • the pharyngeal arches are 1, 2, 3, 4 and 6
  • the 5th pharyngeal arch is rudimental and transient and quickly regresses so it can no longer be seen
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7
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A
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8
Q

What muscles arise from the first pharyngeal arch?

What is their nerve supply?

A
  • 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
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9
Q

What group of muscles form from the second pharyngeal arch?

What is their nerve supply?

A
  • muscles of facial expression
  • the facial nerve supplies all the muscles derived from the second arch
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10
Q

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

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

A
  • 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
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11
Q

What are pharyngeal pouches and how many are present?

When does formation of these pouches occur?

A
  • 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
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12
Q

What is signalling from pouch endoderm important for?

A
  • 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
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13
Q

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?

A
  • 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
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14
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A
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15
Q

How are pharyngeal pouches formed?

What factor is important in this process?

A
  • pharyngeal pouches are formed by migration of endoderm cells laterally
  • this migration is stimulated by fibroblast growth factors (FGFs)
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16
Q

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

A
  • 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 2nd and 3rd pouches
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17
Q

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

What is this process dependent upon?

A
  • 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
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18
Q

What transcription factors are present in the mesenchymal core?

How is this different between the arches?

A
  • 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
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19
Q

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

A
  • 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
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20
Q

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

What factors are important in this process?

A
  • 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
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21
Q

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

A
  • 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
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22
Q

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

A
  • 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
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23
Q

What are the 5 facial prominences mainly derived from?

A
  • 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
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24
Q

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

A
  • 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
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25
Q

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

A
  • 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
26
Q

How is the nasolacrimal duct formed?

A
  • 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
27
Q

What happens following detachment of the nasolacrimal duct?

Where does this duct run to and from?

A
  • 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
28
Q

What forms the intermaxillary segment?

What is it continuous with?

A
  • 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)
29
Q

What are the 3 parts of the intermaxillary segment?

A

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
30
Q

How is the secondary palate formed and what is it formed from?

When does this formation occur?

A
  • the definitive palate is formed from 2 shelf-like outgrowths from the maxillary prominences - the palatine shelves
  • the palatine shelves appear in the 6th 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
31
Q

How is the definitive palate formed?

What delineates the primary and secondary palates?

A
  • 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
32
Q

Why does abnormal facial development usually occur?

What usually occurs alongside this?

A
  • 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
33
Q
A
34
Q

What is the triad associated with Robin sequence?

What type of syndrome is this?

A
  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
35
Q

What can cause Robin sequence?

A
  • 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
36
Q

What causes Treacher-Collins syndrome (mandibulofacial dystosis)?

A
  • 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
37
Q

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

A
  • 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
38
Q

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

A
  • the majority of cases are genetic
  • teratogenic doses of retinoic acid may also be responsible
39
Q

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

A
  • 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
40
Q

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

A
  • 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
41
Q

What causes DiGeorge syndrome?

A
  • 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
42
Q

What is the triad of symptoms associated with DiGeorge syndrome?

A
  1. shortened philtrum
  2. low set ears
  3. nasal clefts
43
Q

What are some other associations of DiGeorge syndrome?

A
  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
44
Q

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

What are the consequences?

A
  • 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
45
Q

Why does defective T-cell function occur in DiGeorge syndrome?

What is this associated with?

A
  • 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
46
Q

Why might there be abnormal midline craniofacial development?

What signalling factor may be implicated?

A
  • 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
47
Q

What are the 3 most common causes of midline disorders?

A
  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
48
Q

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

A
  • 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
49
Q

What are the 3 types of orofacial clefts?

A
  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
50
Q

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

A
  • 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
51
Q

What are examples of an anterior cleft deformity?

What is the underlying cause of this?

A
  • 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
52
Q

What are examples of posterior cleft deformities?

What is the underlying cause?

A
  • 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
53
Q

What is the third type of cleft that can occur?

A
  • it is possible to have a combination of both anterior and posterior clefts
54
Q

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

How is it treated?

A

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
55
Q

What are the 4 main causes of cleft lip +/- palate?

What are the ethnic differences in the presentation of this condition?

A
  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
56
Q

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

A
  • 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
57
Q

Why do oblique facial clefts occur?

A
  • 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
58
Q

What are the problems associated with cleft lip and palate?

A
  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
59
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60
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Embryology (20 decks)

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