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Lecture 21

BIO380H5 Lecture 21: Lecture 21 and 22- Head and Neck -Pgs 294-307-

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Nagham Abdalahad

Neural tube acts as a signaling center which gives rise to the cranial neural crest; the paraxial mesoderm; endoderm of the pharynx and the cranial ectoderm Cranial neural tube becomes segmented due to the Hox gene expression- then neural crest cells leave the neural tube Patterning of the pharyngeal endoderm is based on the exposure to retinoic acid Formation of the first pharyngeal pouch does not require retinoic acid, however, pouch 3 and 4 development do require retinoic acid, whereas, pouch 2 needs some but not much Formation of the pharyngeal arches (neural tube not required) depends on the signals from pharyngeal pouches FGF-8 from the pharyngeal ectoderm is required for the prepatterning of ectoderm Early cranial mesoderm contains paraxial and prechordal mesoderm Paraxial mesoderm- rostral to the occipital somites and is subdivided into somitomeres; connective tissue and skeletal elements form from the messenchymal cells present here Myogenic cells from the paraxial mesoderm undergo migrations to form muscles of the cranial regions Prechordal mesoderm- emits brain inductive signals in the early embryo Lateral plate mesoderm- gives rise to endothelial and smooth muscle cells Development of the Facial Region Messenchyme of the face is neural crest originating from the forebrain to the first two rhombomeres Wnts influence the formation of many facial processes Frontonasal process forms from a signalling system that begins with the synthesis of retinoic acid in a region of ectoderm opposite by the forebrain and continues with the action of shh produced by the ventral forebrain Formation of the Nose and the Olfactory Apparatus At the end of the first month, nasal placodes appear (thickened ectoderm)- require the expression of Pax-6 and originate from the anterolateral edge of the neural plate Nasal placodes form a surface depression called nasal pits Thickened nasal processes (nasomedial and nasolateral) depend on the retinoid-stimulated production of FGF-8 which stimulates the proliferation of messenchymal cells Retinoid acid from the epithelium of the nasal pit Oronasal membrane appears at 6.5 weeks separating the oral cavity from the nasal cavity; soon breaks and makes nasal cavities continuous with the oral cavity via openings behind the primary plate called nasal choanae Nasal conchae form during the third month from the ethmoid bones on the lateral wall of the nasal cavity- increase the surface area available for conditioning the air within the nasal cavity
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