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BIOL 2P95 (35)

Head and Neck Development I

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Dirk De Clercq

Head and Neck Development I. Review a. critical time for craniofacial development is wk 3-4 (same as gastrulation) b. Neural crest cells  bones of face & anterior skull c. Mesoderm  posterior bones & base of skull d. Lateral Plate Mesoderm  laryngeal cartilage II. Somitomeres a. Week 3: paraxial mesoderm differentiates into somitomeres i. differentiate in cranial to caudal fashion ii. 7 somitomeres iii. key in segmenting brain into fore- mid- hind- brain segments by inducing neural tube (via chemogradients) b. somitomere = incompletely differentiated (in head) c. somite = completely differentiated (found in lower body) d. somitomeres form all voluntary muscles in head/neck; but scaffold (patterning) is by connective tissue elements III. Neural Crest Cells and Pharyngeal Arches a. NCCs: migrate from neural folds in cranial-to-caudal sequence to somitomeres pharyngeal arches (brachial arches) i. directed by HOX genes  populate specific regions of head/neck ii. MSX-2 genes: specific “suicide” signal to NCCs along rhombomeres 3 & 5 1. produces gaps (brachial grooves) 2. ensures non-mixing between populations of NCCs iii. Rhombomere: subdivision of hindbrain (segment of neural tube) 1. each rhombomere has its own HOX code 2. NCCs in each rhombomere migrate to brachial arches with compatible HOX code  specific link between face & brain development iv. NCCs provide link between development of face & brain; key BUT also very sensitive to teratogens b. Pharyngeal Arches (Brachial Arches) i. General: th 1. develop during 4 week of gestation 2. lateral swellings on either side of head; result from local proliferation of NCC 3. give rise to specific cartilages and bones 4. pharyngeal grooves—external delineations between brachial arches; ectoderm covered 5. pharyngeal pouches—internal deliniation between brachial arches; endoderm lined 6. closing membranes—ectoderm/endoderm contact points ensuring non-mixing between brachial arches ii. Aortic Arch Arteries 1 1. blood vessel development follows path of least resistance (thru mesenchyme) => many run through brachial arches during early embryogenesis iii. Cranial Nerves—each is associated with a specific pharyngeal arch & innervates all its muscles, etc iv. Muscles—each pharyngeal arch differentiates into a specific set of muscles which are innervated by the CN in that arch Arch Artery Nerve Skeletal Elements Muscles (NCCs) Maxilla, mm. of mastication (temporalis, masseter, 1 Maxillary V(V 2 V 3 mandible, zygomatic pterygoids) myelohyoid, ant. belly of only) portions of temporal, digastric, tensor tympani, & veli palatini Stapedial (NCCs) Stapes, styloid muscles of facial expression, post. belly 2 (practically VII degenerates) process, part of hyoid of digastric, stylohyoid, stapedius 3 Carotid IX (NCCs) Part of hyoid Stylopharyngeus Right: subclavian, X (superior (LPM) Laryngeal Cricothyroid, levator veli palatine, 4 Left: arch of laryngeal br) cartilages pharynx constrictors the aorta 5 - - - - 6 Pulmonary X (recurrent (LPM) Laryngeal Intrinsic larynx mm. laryngeal br) cartilages IV. Pharyngeal Pouches & Grooves a. Pharyngeal Pouches (Glands) i. First Pharyngeal Pouch 1. forms the auditory tube (narrow proximal part) & tympanic cavity (distal sac-like structure) 2. tympanic membrane—forms from lining of tympanic cav. st 3. later comes into contact with epithelial lining of 1 pharyngeal cleft (future EAM) ii. Second Pharyngeal Pouch 1. proliferates to form palatine tonsil iii. Third Pharyngeal Pouch 1. forms thymus and inferior parathyroid glands 2. thymus descends during development, pulling superior/ inferior parathyroid glands with it iv. Fourth Pharyngeal Pouch 1. forms superior parathyroid glands 2. attach to dorsal surface of thyroid v. Fifth Pharyngeal Pouch 1. forms ultimobrachial body—regulates Ca in body & is embedded in thyroid b. Pharyngeal Grooves (Clefts) i. four recognizable clefts; all but one disappear during development 2 ii. 1 pharyngeal cleft  external auditory meatus (EAM) & part of tympanic membrane iii. 2 pharangeal arch mesenchyme proliferates  overlapping arch that covers 2-4 pharangeal clefts  smooth neck iv. cervical sinus—temporary cavity formed during development; closes later during development V. Tongue Development a. Essentially a group of muscles covered by mucosa b. Mucosa th i. Endoderm derived; from 1-4 pharyngeal arches ii. Sensory Innervation: 1. anterior 2/3—mostly 1 brachial arch  CN V rd th 2. Posterior 1/3—3 & 4 pharyngeal arches  CN IX & X iii. Special Sensory (Taste): CNs VII (ant 2/3), IX, & X (post. 1/3) c. Frenulum—band of tissue underlying tongue that anchors it to floor of mouth; extensively degenerates during development d. Terminal sulcus—sulcus separating anterior part of tongue from posterior e. Foramen Cecum—midpoint of sulcus; position of thyroid outgrowth VI. Thyroid Gland Development a. Arises from epithelial proliferation (endoderm) in floor of mouth on midline @ foramen cecum b. Descends along vertical path to above laryngeal cartilages c. Remains connected to tongue via thyroglossal duct (a narrow canal) VII. Facial Development a. Frontonasal Prominences i. Frontonasal prominence—mesenchyme surrounding forebrain ii. Nasal Placode—local thickening of surface ectoderm; develops into medial/lateral nasal prominences (develop into nose) iii. Medial nasal prominence (M
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