Development of the Face & Palate Flashcards
When do the pharyngeal arches begin to form?
How do they develop over the coming weeks to have a characteristic appearance?
- 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
What is significant about the contents of each pharyngeal arch?
- 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
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?
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:
- mandibular prominences (1st pharyngeal arch) caudal to the stomodeum
- maxillary prominences (dorsal portion of 1st pharyngeal arch) lateral to the stomodeum
- frontonasal prominence that is a rounded elevation cranial to the stomodeum
What is the composition of each pharyngeal arch?
- 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
What is formed by the mesenchymal core and NCCs of each pharyngeal arch?
- 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
How many pharyngeal arches are present?
- 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
What muscles arise from the first pharyngeal arch?
What is their nerve supply?
- 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
What group of muscles form from the second pharyngeal arch?
What is their nerve supply?
- muscles of facial expression
- the facial nerve supplies all the muscles derived from the second arch
How many pharyngeal clefts are present in the 5-week embryo?
Which structures are formed from these clefts and how do they develop?
- 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
What are pharyngeal pouches and how many are present?
When does formation of these pouches occur?
- 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
What is signalling from pouch endoderm important for?
- 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
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?
- 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
How are pharyngeal pouches formed?
What factor is important in this process?
- pharyngeal pouches are formed by migration of endoderm cells laterally
- this migration is stimulated by fibroblast growth factors (FGFs)
What expression of genes is exhibited by pharyngeal pouches as they form?
- 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
What is the significance of the characterisitc pattern of gene expression of the pharyngeal arches?
What is this process dependent upon?
- 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
What transcription factors are present in the mesenchymal core?
How is this different between the arches?
- 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
What is the epithelial-mesenchymal interaction involved in skeletal patterning?
- 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
What is formed by the neural crest and mesenchyme within the frontonasal prominence?
What factors are important in this process?
- 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
Which of the 3 facial prominences are formed by the 1st pharyngeal arch?
- 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
How can the 3 facial prominences appearing in the 4th week be distinguished from each other?
- 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
What are the 5 facial prominences mainly derived from?
- 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
What do local thickenings near the frontonasal prominence go on to form?
- 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