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Objectives
After studying this chapter, you should be able to:
Describe the components, development, and derivatives of the pharyngeal apparatus.
Describe the formation of the tongue from the lingual swellings and relate the tongue's innervation to its developmental origins.
Trace the embryonic descent of the thyroid gland.
Describe the formation of the face and palate by the fusion of facial prominences.
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During weeks 4 and 5, a series of ridges develops bilaterally in the region of the primitive pharynx, constituting the pharyngeal (branchial) apparatus (Figure 9-1).
The pharyngeal apparatus has four main components:
Pharyngeal arches: Five thickened ridges
Pharyngeal clefts (grooves): Four indentations between the pharyngeal arches on the external surface of the embryo
Pharyngeal pouches: Four recesses between the pharyngeal arches inside the primitive pharynx
Pharyngeal membranes: Four thin areas between the pharyngeal arches where the pharyngeal clefts and pouches approach each other
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The five pharyngeal arches are numbered 1, 2, 3, 4, and 6 because the fifth arch (which is present in fish and lower vertebrates) either immediately regresses or never forms in humans.
Each arch is covered by ectoderm on its external surface and lined by endoderm on its internal surface.
Each arch contains the following elements:
Mesoderm, which will give rise to muscles
Neural crest cells, which will form the skeletal components of the face and the ganglia of the cranial nerves
A cranial nerve, which will innervate the muscles derived from each arch and carry sensory information to the brainstem
An aortic arch artery (described in Chapter 4)
The muscles of the pharyngeal arches do not always attach to ...