Pharyngeal Arch Development- Biomedicine in Relation to Dentistry II

• What is ankyloglossia?

  1. tongue tied

  2. thick lingual frenum resulting in limitation of tongue movement

  
• Where are the muscles of the tongue derived from?

  Occipital somites
• What is the tongue innervated by?

  
• What is the tongue derived from?

  Pharyngeal arches 1-4

  
• When does tongue development start?

  4th week of gestation
• What does a thyroglossal duct cyst look like?

  
• What do the C-cells of the thyroid do?

  Secrete calcitonin, regulating Ca2+ levels in blood
• What does pharyngeal pouch 4 develop into?

  Dorsal portion: forms superior parathyroid glands (chief & oxyphil cells are derived from endodermal lining of pouch)Ventral portion:ultimobranchial body, which is incorporated into thyroid. Forms the parafollicular or C-cells of thyroid
• How do T cell progenitors from the bone marrow establish the thymus?

  They infiltrate the cortex
• What does pharyngeal pouch 3 develop into?

  Dorsal portion: forms inferior parathyroid glands (chief & oxyphil cells are derived from endodermal lining of pouch)

  Ventral portion: forms thymus (epithelial reticular cells are derived from endodermal lining of pouch)
• What does pharyngeal pouch 2 develop into?

  Forms numerous infoldings that become the crypts of the palatine tonsil
• What does pharyngeal pouch 1 develop into?

  Auditory tube & middle ear cavity
• What are pharyngeal pouches lined by and what is their importance?

  Lined by endoderm and they all form important structures in the adult
• What can happen if some remnants of clefts are left?

  It can result in cysts
• What happens to the other clefts during development?

  They get obliterated
• How does the first pharyngeal cleft develop?

  In 3rd month, ectoderm at bottom of cleft between 1st & 2nd arch proliferates

  Forms solid epithelial plate of cells, 'meatal plug'

  7th month, plug disintegrates
• What is the importance of the first pharyngeal cleft?

  It's the only one that forms a structure in the adult: the first cleft between the 1st and 2nd arch forms the external auditory tube
• Give some examples of clinical features of first & second pharyngeal arch syndromes.

  
• What is Nager syndrome (acro-facial dysostosis) characterised by?

  No angle between neck & chin

  Often associated with cleft palate

  Forehead may be prominent

  Could have cardiovascular abnormalities
• What is Treacher-Collins syndrome (mandibulofacial dysostosis) and what is it characterised by?

  First arch neural crest fails to migrate properly so can result in mandibular hypoplasia, facial abnormalities e.g.

  underdeveloped facial bones, very small lower jaw & chin (micrognathia)
• What are the different types of clefts?

  
• What are some potential reasons for cleft palate arising?

  Inadequate growth of palatine shelves

  Failure of shelves to elevate above tongue

  Excessively wide head

  Failure to fuse

  Secondary rupture after fusion

  (normally corrected surgically between 3-8 months of age)
• How common is cleft palate?

  Found in ~1/2000 live births (more often in females) & is often associated (50%) with cleft lip
• When does cleft palate occur?

  When the palatal shelves fails to fuse with each other during the 7th to 10th weeks of development
• How do the palatine shelves initially grow?

  They initially grow beneath the tongue but soon move upwards to meet at the midline above the tongue: here they fuse with each other & the nasal septum. Lower half of palate undergoes ossification, forming the hard palate. Fusion of palatine shelves occurs 1 week earlier in male compared to female embryos
• What happens in late 7th/8th week of development, around the same time that the palate is forming?

  Growth of the nasal septum separates the left & right nasal chambers
• What is the secondary palate formed from?

  Forms from the palatine shelves (maxillary prominences) that grow medially from the maxillary prominences
• What is the primary palate formed from?

  An extension of the intermaxillary process
• What is the palate?

  The tissue intervening between the nasal & oral cavities
• What are the lower lip and jaw formed by?

  Mandibular processes
• What does the lateral nasal process form?

  The alar (Alae) structures of the nose
• What happens during the 5th & 6th weeks of development?

  Bilateral maxillary processes derived from 1st brachial arch fuse with medial nasal process to form upper lip, alveolus & primary palate
• Summarise the contributions (of the pharyngeal arches) to cartilage, muscle and nerve.

  
• Where is the nerve supply to the third arch from?

  Glossopharyngeal nerve (C.IX)
• What muscle arises from the third arch?

  Stylopharyngeus muscle
• What is the structure of the third arch?

  The cartilage forms the greater horns & lower part of hyoid bone
• Which muscles are included in the second arch?

  Muscles of facial expression
• Where is the nerve supply to the second arch from?

  Facial (C.VII)
• Describe the structure of the hyoid arch.

  Cartilage forms the stapes, styloid process on base of skull, and lesser horns & upper part of the hyoid bone
• What is the second arch known as?

  Hyoid arch