Head and Neck Development (Week 1--Trelease) Flashcards
When do the face and important structures of the head and neck develop from pharyngeal arch tissues?
Between 4th and 12th embryonic weeks
What does the pharynx look like during the 4th and 5th week of development?
Primordial pharynx is bounded laterally by pharyngeal (branchial) arches
Pharyngeal apparatus
Arches, grooves/clefts (separate arches externally), pouches (separate arches internally) make up the pharyngeal apparatus
Pharyngeal apparatus is transformed into face, lips, jaws, tongue, palate, pharynx, etc
Pharyngeal arch components
Ectoderm
Nerve
Cartilage
Artery
Endoderm
Clefts (grooves)
Pouches
What do neural crest cells do regarding pharyngeal arches?
Neural crest cells migrate into each arch and are the source of connective tissue components (cartilage, bone, ligaments, nerves)
What arteries are derived from what pharyngeal arches?
Arch 1: mostly disappears, maxillary artery and part of external carotid artery remains
Arch 2: mostly disappears, hyoid artery and stapedial artery remain
Arch 3: forms common carotid and initial internal carotid artery
Arch 4: forms right subclavian artery, left arch of aorta
Neural crest cells give rise to which ganglia and peripheral nerves?
Trigeminal ganglion
CN V (V1, V2, V3)
CN VII (facial nerve)
CN IX (glossopharyngeal nerve)
CN X (vagus nerve)
Neural crest cells from which pharyngeal arches form which cargilages and eventually which bones?
Arch 1: Meckel’s cartilage –> maxilla, mandible, zygoma, part of temporal bone, malleus, incus, several ligaments
Arch 2: Reichert’s cartilage –> styloid process, upper part of body of hyoid bone, lesser horn of hyoid bone
Arch 3: cartilage –> lower part of body of hyoid bone, greater horn of hyoid bone
Arch 4-6: cartilage –> laryngeal cartilages (thyroid, cricoid, arytenoid, corniculate, cuneiform)
What do the pharyngeal arch grooves/clefts and pouches (internal) turn into?
Arch 1: tubotympanic recess –> auditory (Eustachian) tube, middle ear cavity, mastoid sinus, inner part of tympanic membrane
Arch 2: tonsillar fossa and palatine tonsil (+ migrating mesoderm)
Arch 3: inferior parathyroid glands and thymus
Arch 4-6: superior parathyroid glands and ultimobranchial body, which fuses with thyroid and forms parafollicular C (calcitonin secreting) cells (lower arch 4)
Do the pharyngeal grooves (external) and pharyngeal membranes all disappear?
All except for the first pair of pharyngeal grooves and membranes disappear
First pharyngeal groove –> external acoustic meati
First pharyngeal membrane –> tympanic membranes
What forms the cervical sinus and cysts?
Ectodermally lined remnants of clefts 2-4 may persist as a cervical sinus, with cervical (or branchial) cysts
Cysts may drain to the surface or internally via fistulae
What parts of the tongue develop from what pharyngeal arches?
Tuberculum impar (medial lingual swelling) from arch 1
Copula (hypobranchial eminence) from arches 2, 3, 4
Epiglottal swelling forms from arch 4
Lateral swellings overgrow tuberculum and fuse to form anterior 2/3 of tongue (still arch 1 innervation from V3)
Posterior 1/3 of tongue forms copula and arch 3 overgrows arch 2 so innervation from CN IX
How do we explain CN XII motor innervation of the tongue?
Most tongue myoblasts thought to originate from occipital myotomes (although some may be developed in situ), so get innervation from CN XII
How does the thyroid develop?
Epithelium proliferates in pharyngeal floor at position of later foramen cecum (between tuberculum impar and copula)
Developing thyroid tissue becomes bilobed and descends ventral to gut but remains connected to surface of tongue by thyroglossal duct
Thyroid lobes descend anterior to hyoid bone and laryngeal cartilages and reach final position anterior to trachea in week 7
Thyroid function begins around end of 4th month
Thyroglossal cysts
Congenital thyroglossal cysts may appear anywhere along the thyroglossal duct, about half near hyoid body
Cysts can rupture, forming fistulae
Any aberrent thyroid tissue may be found along migratory course
End of 4th week development of midface and jaw from arch 1
By end of 4th week, facial prominences have been formed by arch 1 and neural crest mesenchyme
Maxillary prominences lateral to stomodeum (primitive oral opening)
Mandibular prominences caudal to stomodeum
Frontonasal prominence formed by mesenchyme ventral to brain vesicles, rostral to stomodeum
Nasal (olfactory) placodes formed on frontonasal processes
5th week development of nasal primordia
Nasal placodes invaginate to form nasal pits with nasal ridges
Lateral and medial nasal prominences
6th and 7th week development of midface and jaw from arch 1
Maxillary prominences grow and push medial nasal prominences toward midline
Clefts between medial nasal prominences and maxillary prominences close, forming upper lip
Lateral nasal prominences do NOT contribute to upper lip
Fusion of mandibular processes across midline form lower lip and jaw
Nose forms from the 5 facial prominences (frontal prominence forms bridge, medial prominences form crest and tip, lateral prominences form alae)
Medial nasal prominences merge deeply to form upper jaw component (4 incisor/front teeth) and primary palatal component (altogether is intermaxillary segment)
What is anteromedial primary palate formed from?
Intermaxillary segment (frontonasal process origin)
What is main part of palate formed by?
2 internal shelf-like outgrowths called maxillary palatine shelves (at 6.5 weeks)
Initially slope obliquely to side of tongue
Become horizontal above tongue and begin to close to form secondary palate (7.5 weeks)
Fusion of palatine shelves and primary palate (10 weeks)
Developmental anomalies of face and palate
Common
Mainly result of maldevelopment of neural crest tissue (deficiency in # of cells, incomplete migration into face, failure of induction)
Arrested development or failure of fusion of facial and palatal prominences, or both
Thought to involve a combination of genetic and environmental factors
Cleft lip
Failure of merger of mesenchymal masses in medial nasal and maxillary prominence
(early on, around 7 weeks?!)
Cleft palate
Failure of mesenchymal masses of palatine shelves (processes) to meet and to merge
(later on, 7-10 weeks is when they’re supposed to close?!)
In general, what causes facial clefts?
Neural crest migration failure due to delay or low cell numbers
Multifactorial process (genes and environment)
Which pharyngeal arches form which cranial nerves?
Arch 1: CN V
Arch 2: CN VII
Arch 3: CN IX
Arch 4 and 6: CN X