Head and Neck Embryology Flashcards
Ectoderm
nervous system, skin (epidermis and appendages), neural crest cells and derivatives
Mesoderm
bone, cartilage, muscles, connective tissue (dermis), dura mater, heart, vessels, blood, reproductive organs, genitourinary system
Endoderm
GI and respiratory lining, digestive organ parenchyma
Neural crest cells
Ectodermal origin; pluripotent; migrate along cleavage planes, differentiate into connective, muscle, nervous, endocrine, and pigmentary tissues, induce differentiation of the tissue they invade
Pharyngeal (branchial) arches (definition and origin, not specific derivations)
from migrating NCCs and surrounding pharyngeal endoderm and mesoderm; different from somites; separated by pharyngeal grooves on the external surface and pharyngeal pouches on the internal surface; grooves and pouches are separated by mesoderm; each arch has nervous, arterial, muscular and bony components
Somites
mesodermal swellings around the neural tube
Pharyngeal/Branchial Arch I
N: CN V
A: Maxillary artery
B: Greater wing of sphenoid, malleus, maxilla, zygomatic, temporal (squamous), mandible
M: Muscles of mastication, anterior digastric, mylohyoid, tensor tympani, tensor veli palatini
Ear: forms anterior hillocks (tragus, root of helix, superior helix)
Pharyngeal/Branchial Arch II
N: CN VII
A: Stapedial artery (corticotympanic)
B: Stapes, styloid process, stylohyoid ligament, lesser horn and upper body of horn
M: Muscles of facial expression, posterior digastric, stylohyoid, stapedius
Ear: posterior hillocks (antitragus, antihelix and lobule)
Pharyngeal/Branchial Arch III
N: CN IX
A: Common carotid, proximal internal carotid
B: Greater horns and lower body of hyoid
M: Stylopharyngeus
Pharyngeal/Branchial Arch IV/VI
N: CN X
A: Aortic arch, right subclavian, origin of pulmonary arteries, ductus arteriosus
B: laryngeal cartilages
M: Pharyngeal constrictors, levator veli palatini, palatoglossus, striated upper esophageal muscles, laryngeal muscles
Pharyngeal Groove(Cleft) I
becomes external auditory canal, mesoderm becomes tympanic membrane
Pharyngeal Groove II-IV
operculum flap grows downward from arch II and fuses below cleft IV to create cervical sinus
Anomalies from groove II are the most common
Pharyngeal grooves (clefts)
Failure to obliterate can lead to cysts (sealed in neck), sinuses (end in blind sac) or fistulas (connect with pharynx; often detected in the second decade of life and palpable at the anterior border of the SCM
Pharyngeal Pouch I
Internal auditory canal
Pharyngeal Pouch II
Palatine tonsil
Pharyngeal Pouch III
inferior parathyroid and thymus
Pharyngeal Pouch IV
Superior parathyroid (migrates above pouch III)
Pharyngeal Pouch V
Ultimobranchial body (thyroid C cells)
Intramembranous ossification
cartilaginous precursors resorb; mesenchymal cells directly differentiate into osteoblasts without cartilaginous intermediate
Endochondral ossification
cartilaginous template is directly and gradually replaced with a bony matrix
Neurocranium
portion of the skull encasing and protecting the brain
Membranous neurocranium
forms via intramembranous ossification of neural crest origin, includes paired frontal, squamosal, and parietal bones, and upper occipital bone
Cartilaginous neurocranium
forms via endochondral ossification of mesodermal origin, includes sphenoid and ethmoid bones, mastoid and petrous temporal bone, and the base of the occipital bone
Viscerocranium
bones of the facial skeleton; forms via intramembranous ossification of pharyngeal arch I, except for Meckel’s cartilage (which forms the malleus and mandibular condyles)
Craniosynostosis
sutures fuse prematurely, associated with FGFR and TGFßR and TWIST and Wnt pathways
Virchow’s Law (of skull)
After suture fusion, growth proceeds parallel to suture instead of perpendicular
Five Prominences of the Face
Frontonasal (1), Maxillary (2), Mandibular (2)
Frontonasal prominence
pulled ventrally and caudally, forming the forehead, nasal dorsum (apex), and medial and lateral nasal prominences
Maxillary prominence
Migrate medially to form the secondary palate, lateral maxilla, and lateral lip
Tessier number 3
Oblique facial cleft; Failure of fusion of the junction with the lateral nasal prominences forms the nasolacrimal groove and nasolacrimal duct system
Mandibular prominence
Forms mandible, lower lip, and lower face
Prominent growth factors and signaling pathways
TGFß, BMP, FGF, IRF, Wnt, FOXe1
Teratogens affecting Sonic Hedgehog Pathway
retinoids, alcohol, drugs that affect cholesterol synthesis and transport
Stomodeum
primitive mouth forms at 3-4 weeks from invagination of the ectoderm around the buccopharyngeal membrane
Unilateral cleft lips
result from failure of fusion of the medial nasal prominence and a maxillary prominence
Tessier Cleft 7
Cleft at the lateral oral commissure, results from failure of fusion of the maxillary and mandibular prominences
Tessier Cleft 0
median cleft lips are rare and result from failure of the medial nasal prominences to fuse
Tessier Cleft ?
central defects of the lower lip and chin result from failure of the mandibular processes to fuse
Primary Palate and Cleft
5-6 weeks, medial nasal prominences come together to form the primary palate, midmaxilla, and septum; cleft results from failure of fusion of the medial and lateral palatine processes
Secondary Palate and Cleft
9-12 weeks, lateral palatine shelves initially hang vertically but assume a horizontal position as the tongue drops with mandibular growth; right palate drops first, which explains higher incidence of cleft palate on the left side; cleft results from failure of fusion of the lateral palatine process with the nasal septum
Incisive foramen
Lies between the primary and secondary palate
Epstein pearls
along the median raphe or junction of the hard/soft palates result from cystic degeneration of epithelial lining at edges
Nasopalatine duct cysts
at the incisive foramen result from epithelial entrapment at the junction of developing primary/secondary palates
Thyroid
from endodermal proliferation at foramen cecum of tongue, descends with a trailing diverticulum to final position distal to cricoid cartilage
Thyroglossal duct cysts
may form anywhere along the path of the thyroid, presenting as painless midline (or near midline) neck mass, may rupture and result in sinus or fistula formation; treat with sistrunk procedure (removal of cyst and central hyoid bone)
Lingual thyroids
result from failure of thyroid descent
Anterior 2/3 of tongue
Originates from pharyngeal arch I and is innervated by lingual nerve CN V3
Posterior 1/3 of tongue
Originates from arches III and IV (these overtake arch II) and is innervated by CN IX and X
Tongue muscles
Arise largely from occipital myotomes (CN XII) except for palatoglossus (CN X)
External Ear
Forms at interface between pharyngeal arches I and II
Arch I
Forms anterior hillocks (tragus, root of helix, and superior helix)
Arch II
Forms three posterior hillocks (antitragus, antihelix, lobule)
Groove I
lies between arches I and II and forms external auditory canal
Medial nasal prominence
Primary palate, midmaxilla, midlip, philtrum, central nose, and septum
Lateral nasal prominence
Nasal ala
