Face/Palate & Pharyngeal derivatives

• Palate development

  1. Median palatal process

  2. Secondary palate
• Facial clefts

  • Cleft lip
  • Incomplete cleft lip
  • Unilateral cleft lip
  • Bilateral cleft lip
  • Cleft palate
  • Median cleft lip
  • Oblique facial clefts
• Facial prominences

  • The development of the face takes place between the 4th and 8th weeks of embryonic development
  • It is only at the end of the fetal period that the face will become human-looking
• Development of the facial prominences

  1. By the end of the 4th week, the facial prominences will be visible

  2. Caudal to the stomodeum, there are 2 mandibular prominences which later fuse to form the mandible

  3. Cranial to the stomodeum is the frontonasal prominence

  4. On both sides of the frontonasal prominence, there are depressions of ectoderm (known as the nasal placodes)

  5. The nasal pits will invaginate further, such that a ridge of tissue is created around them

  6. The nasal prominences are known as the lateral and medial nasal prominences on each side

  7. The frontonasal prominence changes its name to the frontal process

  8. The maxillary prominences will continue to increase in size, and as they do, they will compress the 2 medial nasal prominences towards each other in the midline of the face

  9. The two medial nasal prominences fuse to form a single median nasal prominence

  10. The cleft between the between the maxillary prominence and the median nasal prominence will be lost on either side

  11. The median nasal prominence will form the bridge of the nose, the nasal septum, the columella, the philtrum of the lip, and the frenulum on the inside of the lip

  12. The maxillary prominences will then enlarge to form the cheeks and the maxilla
• Prominence

  Same as process
• Development of the nose

  1. The frontal prominence will give rise to the bridge of the nose

  2. The median nasal prominence will give rise to the crest and tip of the nose

  3. The lateral nasal prominences form the sides of the nose
• Nasolacrimal duct formation

  1. Initially, the nasolacrimal groove separates the lateral nasal prominence from the maxillary prominences

  2. There's a cleft formed by their groove initially, and the ectoderm on the floor of this groove will later form an epithelial cord of cells, which will then detach from the overlying ectoderm

  3. This epithelial chord of cells will later canalize to form the nasolacrimal duct

  4. Following the formation of the duct, these clefts will be lost

  5. The upper part of the nasolacrimal duct will dilate to form the lacrimal sac which opens into the inferior meatus of the nasal cavity
• Palate formation

  1. The medial nasal prominence will form a triangular mass of mesenchyme behind the philtrum of the lip, known as the median palatal process, or primary palate

  2. Each maxillary prominence will give rise to 2 projections known as palatine shelves, which are projected vertically on the sides of the tongue

  3. Later, they will elevate to lie horizontally above the tongue, where they will fuse

  4. These are the two palatine shelves, which have now elevated in a horizontal manner and are moving towards each other to fuse in the midline of the face

  5. As they fuse, they will merge with the median palatal process to form the definite palate

  6. The fusion of these two palatine shelves is facilitated by a sticky substance known as glycocalyx which is present in the epithelial surface of these palatine shelves

  
• Facial clefts

  • Unilateral cleft lip: Failure of the maxillary prominence to fuse with the median nasal prominence
  • Unilateral cleft lip and palate: Cleft palate is at the level of the median palatal process
  • Bilateral cleft lip and palate: Cleft palate is at the level of the median palatal process, The palatine shelves did not fuse with the median palatal process
  • Isolated median cleft palate: Failure of the two palatine shelves to fuse
  • Cleft palate: Involves the palatine shelves, Associated with a unilateral cleft lip
• During the 4th week of development, the embryo looks like that of a fish at an equivalent stage due to the presence of the pharyngeal arches
• The pharyngeal arches appear in a cranio-caudal sequence, which means that the first two pharyngeal arches form first, and the latter subsequently
• Origin of the pharyngeal arches

  1. The stomodeum is the future oral cavity, lined by ectoderm

  2. The pharyngeal gut is the most anterior part of the gut

  3. The oropharyngeal membrane: When it ruptures, the cavity of the stomodeum will become confluent with the pharyngeal gut

  4. The Rathkes pouch/pocket is important later in the development of the pituitary gland

  5. The mesoderm is populated by neural crest cells, which will arrange itself into segments with depressions in between, which will become pharyngeal arches

  6. There are 6 pharyngeal arches which form, but the 5th is a rudimentary structure
• Pharyngeal arches in cross section

  • Each pharyngeal arch consists of a core of mesoderm which is lined on the outside by ectoderm, and by endoderm on the inside
  • The pharyngeal arches are separated from each other by depressions on the inside and outside
  • The depressions on the outside – which are ectodermal in origin – are the pharyngeal clefts/grooves
  • The inside depressions are pharyngeal pouches
  • Each pouch, groove and core of mesenchyme has its own derivatives
• Derivatives of the mesenchyme in the pharyngeal arches

  1. The mesenchyme in each pharyngeal arch will condense to form a cartilage, which is known as the pharyngeal arch cartilage

  2. Caudal to this, the mesenchyme will differentiate to form the aortic arch artery aka pharyngeal arch artery

  3. Some mesenchymal cells will differentiate into striated muscle
• Nerves in the pharyngeal arches

  • Coming from the hindbrain and growing into these pharyngeal arches is a mixed nerve consisting of a motor and sensory component
  • The motor component will supply the striated muscles of a particular pharyngeal arch
  • The sensory component divides into 2 parts: Posttrematic branch and Pretrematic part/nerve
• First pharyngeal arch

  1. Consists of the dorsal maxillary process and ventral mandibular process

  2. Mandibular prominence contains Meckel's cartilage

  3. The maxillary prominence gives rise to premaxilla, maxilla, zygomatic bone and part of temporal bone

  4. Muscles include muscles of mastication, anterior belly of digastric, mylohyoid, tensor tympani, and tensor palatini

  5. Muscle innervation is by the mandibular branch of the trigeminal nerve

  6. The artery of this arch is the primitive maxillary artery
• Second pharyngeal arch

  1. Contains cartilage (of the Hyoid arch)

  2. This cartilage gives rise to stapes, styloid process of the temporal bone, stylohyoid ligament, the lesser horn and upper part of the body of the hyoid bone

  3. Muscles include the stapedius, stylohyoid, posterior belly of the digastric, auricular, and muscles of facial expression

  4. These muscles are supplied by the facial nerve

  5. Stapedial artery
• Third pharyngeal arch

  1. Produces lower part of the body of the hyoid bone

  2. Contributes to the mucosa of the posterior third of the tongue

  3. Stylopharyngeus muscles

  4. The muscles are innervated by the glossopharyngeal nerve

  5. Internal carotid artery
• Fourth and sixth pharyngeal arches

  • Cartilage components fuse to form the thyroid, cricoid, arytenoids, corniculate, and cuneiform cartilages of the larynx
  • Muscles (cricothyroid, levator palatine, constrictors of pharynx) are innervated by the superior laryngeal branch of the vagus (superior part of the accessory nerve)
  • Intrinsic muscles of the larynx are supplied by the recurrent laryngeal branch of the vagus (superior part of the accessory nerve)
• These two branches of the vagus nerve function as conduits because they carry fibres of the superior part of the accessory nerve, which in fact supplies the 4th & 6th pharyngeal arches
• Arteries of the 4th pharyngeal arch

  • Left: aortic arch
  • Right: proximal part of the right subclavian
• Arteries of the 6th pharyngeal arch

  • Pulmonary arteries
  • Ductus arteriosus
• Fifth pharyngeal arch is a rudimentary structure and does not have any adult derivatives
• Development of the pharyngeal pouches

  • The pouches are found on the inside of the pharyngeal arches and are endodermal in origin
  • Each pouch will develop a dorsal and ventral diverticulum except for the 1st pharyngeal pouch
  • The 1st pharyngeal pouch only develops a dorsal diverticulum (the tongue forms in the region where the ventral diverticulum would form)
• Derivatives of the 1st pharyngeal pouch

  • Forms the tubotympanic recess, which dilates at its distal end to form a primitive tympanic cavity
  • The proximal part will form the auditory tube
  • The lining of the tympanic cavity will contribute towards the formation of the eardrum
• Derivatives of the 2nd pharyngeal pouch

  • The ventral lining will proliferate and differentiate into the palatine tonsil (dorsal portion will be discussed later in the ear)
• Derivatives of the 3rd pharyngeal pouch

  • Dorsal part forms the inferior parathyroid gland – which loses its connection to the pharyngeal gut, and will populate the migrating thyroid gland
  • The ventral part of the 3rd pharyngeal pouch will form the thymus
  • The thymus will migrate in a caudal and medial direction, pulling the inferior parathyroid gland with it
• Derivatives of the 4th and 5th pharyngeal pouches

  • The 4th pharyngeal pouch also has a dorsal and ventral diverticulum, but it is unknown what the ventral one forms
  • The dorsal diverticulum forms the superior parathyroid gland, which also loses its connection with the pharyngeal gut and populates the superior part of the migrating thyroid gland
  • The 5th pharyngeal pouch (rudimentary) consists of the ultimobranchial body, which does not form any adult structure but is the resting place for neural crest cells which will later form the parafollicular cells aka C cells of the thyroid gland
• Derivatives of the pharyngeal grooves/clefts

  • During the 5th week of development, four pharyngeal grooves (PG 1, 2, 3, 4) are evident, but only one (1st PG) will result in a definitive structure: the external acoustic meatus
  • 2nd pharyngeal arch will expand in a lateral and caudal region to fuse with the epicardial ridge in the lower regions of the neck, overlapping and obliterating PG's 2, 3 & 4
  • These pharyngeal grooves will form a cavity known as the cervical sinus, which disappears with further development
• Branchial sinuses and fistula

  • Occur when pharyngeal grooves 2, 3 and 4 persist
• Tongue

  • Consists of two parts: the anterior 2/3 (body) and posterior 1/3
  • Divided by the V-shaped sulcus terminalis

  
• Development of the tongue body

  1. Formed by 3 dilatations derived from the 1st pharyngeal arch - 2 lateral lingual swellings & 1 median tuberculum impar

  2. These three structures fuse to form the body of the tongue

  
• Tuberculum impar doesn't grow as much as the two lateral swellings


  The lateral lingual swellings will fuse anterior to it
• Innervation of the tongue body

  Supplied by the trigeminal nerve (mandibular branch) and the chorda tympani nerve of the facial nerve

  
• Posterior 1/3 of the tongue

  • Formed by the copula, which is a fusion of pharyngeal arches 2,3 and 4
  • The contribution of the 2nd pharyngeal arch will be submerged by the mesenchyme of the 3rd & 4th pharyngeal arches
• Innervation of the posterior 1/3 of the tongue

  Glossopharyngeal nerve (nerve of the 3rd pharyngeal arch) and the superior laryngeal nerve of the vagus (nerve of the 4th pharyngeal arch)

  
• Muscles of the tongue

  • Derived from the occipital myotomes
  • Myotomes migrate from the somites to populate the tongue, bringing the hypoglossal nerve with them

  
• Development of the thyroid gland

  • Forms on the ventral floor of the pharynx in the foramen cecum
  • Derived from endoderm
  • Develops from the evagination of the foramen cecum, with the invagination forming the thyroglossal duct
  • The thyroglossal duct connects the developing tongue to the developing thyroid gland
  • The thyroglossal duct develops a bifid tip, forming the two lobes of the thyroid gland joined by an isthmus
  • Reaches its adult position anterior to the tracheal rings by the 7th week of development
• Thyroglossal cysts

  • Occur if the thyroglossal duct does not degenerate completely, can lie at any point along the migratory pathway of the thyroid gland
• Lingual thyroid

  When the thyroid gland fails to migrate from the tongue and remains there