head and neck

Embryology: Head and Neck Begins at start of fourth week Neural crest (cells at junction between surface ectoderm and neural plate) deposits in cranial region as neural folds form • mesenchyme of head: paraxial mesoderm (somitomeres and somites) + neural crest • while in neural folds, neural crest cells receive precise info (via HOX gene code) as to where to go after they leavepopulate very specific region of head • some neural crest cells (along rhombomere 3 and 5), receive instructions from MSX-2 genes to commit suicide to produce gaps to prevent mixing of neural crest cell population • free gaps produce clefts between the upper arches = branchial grooves or branchial clefts neural crest = bones of face and anterior skull mesoderm: posterior bones and base of skull laryngeal cartilages in neck: lateral plate mesoderm mesoderm from primitive streak accumulates on both sides of midline; head region: not completely segmentedsomitomeres • seven somitomeres • form from cranial to caudal sequence • from region of prosencephalon to occipital region • each one associated with specific portion of cranial neural plate; forebrain, midbrain and hindbrain presaged by one or more somitomeres o somitomeres tell neural tube segment what to become • somitomeres set up initial organization of brain and form all voluntary muscle of head and neck • scaffold or patterning of muscles determined by connective tissue of head a neck (neural crest) somitomere primitive brain subdivision cranial nerve craniofacial muscle Origin Craniofacial muscles Innvervation Somitomere 1,2 Sup., med., ventral recti CN III Somitomere 3 Sup. Oblique CN IV Somitomere 4 Jaw-closing mm. CN V Somitomere 5 Lat. Rectus CN VI Somitomere 6 Jaw-opening mm. CN VII nd Other 2 arch mm. Somitomere 7 Stylopharyngeus CN IX Somites 1,2 Intrinsic laryngeals CN X Somites 2-5 Tongue mm. CN XII (Note: Raymond has replaced a table image with equivalent text) hindbrain undergoes further segmentation: rhombomeres; each has unique genetic identity pharyngeal (branchial, visceral) arches (six arches, #5 is degenerate) • lateral swellings on either side of head th • apparent during 5 week • result of local proliferation of neural crest cell populations as they take position • made up of mesenchyme • delineated from each other by pharyngeal grooves, covered by ectoderm (external) and by pharyngeal pouches, lined by endoderm (internal) • closing membranes (ectoderm/endoderm contact points) seal off archesno mixing • each arch has unique genetic identityspecific cartilages and bones • with connective tissue in place, somitomeres and occipital somites can organize the formation of the voluntary musculature associate with their respective connective tissues Summary of info, thus far: • each segment of neural tube has unique genetic identity (HOX)carried into pharyngeal arches through neural crest cellssome cells die to for grooves • coordinate brain and facial development Aortic arch arteries • pharyngeal arches initially serve as conduits for aortic arch arteries o arteries migrate thru path of least resistance: mesenchyme • early in embryogenesis, pattern of arterial supply is symmetrical, but each pair of arch arteries has different fates; some arteries grow, some degenerate • (Note: Raymond has replaced a table image with equivalent text below) Aortic arch 1t Maxillary a. 2d Stapedius a. 3d Carotid a. th Proximal subclavian a. (right), aortic arch (left) 5 None 6th Pulmonary a. Cranial Nerve • each pharyngeal arch associated with specific cranial nerve • CN emerge from specific regions of brain and supply pharyngeal arch with neural crest that came from same region (same HOX code) • To know which arch a muscle came from, know the CN (Raymond has removed a table image from the original document) Pharyngeal pouchesglands/organs • 5 pairs • first pouch o auditory tube o comes in contact with epithelial lining of first pharyngeal cleftfuture EAM o distal portiontympanic cavity (lining will become eardrum) o proximal portionauditory tube • second pouch o forms buds that penetrate surrounding mesenchyme o palatine tonsil o structure infiltrated by lymphatic tissue • third pouch o forms thymus and inferior parathyroid glands o as thymus descends into thoracic region, pulls inferior parathyroid glands with it o inferior parathyroid glands become embedded into gland • fourth pouch o superior parathyroid glands o as thyroid gland moves caudally, glands attach to dorsal surface • fifth pouch o utlimobranchial body o regulates calcium levels in blood o embedded into thyroid gland when pulled caudally pharyngeal clefts/grooves • initially, four clefts exist • only one gives rise to definite structure • 1 pharyngeal cleft penetrates underlying mesenchyme and forms EAM o bottom of EAM forms lateral aspect of tympanic cavity • 2 cleft o undergoes active proliferation and overlaps remaining clefts o merges with ectoderm of lower neckremaining cleft have no contact with outside o temporarily, clefts form cervical sinus, but disappears Tongue development • tongue=groupthf muscles covered by mucosa • appears at 4 week • formed by endoderm and mesoderm of 1 – 4 archth • mesoderm from occipital somites (NOT somitomeres); precursor muscle cells migrate to region of tongue; innervated by GSE component of XII cranial nerve • mucosa: anterior endoderm lining the first four pharyngeal arches; innervation depends on arch derivation o anterior and posterior separated by a terminal sulcus • midpoint of sulcus: foramen cecum position of thyroid outgrowth o mucosa of anterior 2/3 of tongue comes from first archCN V o mucosa of posterior 1/3 of tongue comes from third and 4 archCN IX, X o special taste of anterior 2/3CN VII o Special taste of posterior 1/3CN X • Tongue freed from floor of mouth by extensive degeneration of underlying tissue o Midline frenulum anchors tongue to floor of mouth Thyroid Gland • Arises from foramen cecum • Descends along front of pharyngeal gut • Remains connected to tongue by narrow canal: thyroglossal duct (obliterated later) • Descends just caudal to laryngeal cartilages • Functions during early fetal period Facial development • Nose o At time of anterior neural tube closure, mesenchyme around forebrain, frontonasal prominence (FNP), has smooth rounded extended contour o Nasal placodes (thickening of surface ectoderm to become peripheral neural tissue) develop on frontolateral aspects of FNP o Mesenchyme swells around nasal placodemedial and lateral nasal prominence (nasomedial and nasolateral processes) o Nasal prominencesnose • Mouth o Stomadeum (primitive oral cavity) forms between frontonasal prominence and first pharyngeal arch o First pharyngeal archdorsal maxillary prominence and ventral mandibular prominence o Maxillary prominence will merge with medial nasal prominences, pushing them closer to cause fusion  Fused medial nasal prominences will form midline of nose and midline of upper lip (philtrum) and primary palate (carries first four teeth) • Nasolacrimal structures o Maxillary and lateral nasal prominences separated by deep furrow: nasolacrimal groove o Ectoderm in floor of groove forms epithelial cord, detaches from overlying ectoderm  Epithelial cord canalizesnasolacrimal duct  Upper end widenslacrimal sac o After detachment of cord, maxillary and lateral nasal prominences merge with each othernasolacrimal duct runs from medial corner of eye to inferior meatus of nasal cavity o Maxillary prominences enlargecheeks and maxillae o Lateral nasal prominencesalae of nose Secondary Palate development • Main part of definitive palate formed by two shelf-like outgrowths from maxillary prominences • Outgrowths=palatine shelves th o Appear during 6 week o Directly obliquely downward on each side of tongue—moves down when mandible gets bigger o 7 week: ascend to attain horizontal position , fuse  secondary palate o anteriorly: shelves fuse with triangular primary palate (front 4 teeth) o incisive foramen: midline landmark b/w primary and secondary palate o at time palatine shelves fuse, nasal septum (outgrowth of median tissue of frontonasal prominence) grows down and joins cephalic aspect of newly formed palate Ear development • three distinct parts