Lecture Tuesday September 25: head and neck embryology I

A. Pharyngeal Arches
Be able to identify the pharyngeal arches in a picture or drawing of an embryo and what things are derived from them.



Arch 1: muscles of mastication. malleus and incus of the ear. Innervation by V3

Arch 2: muscles of facial expression. stapes, styloid process and upper part of hyoid bone. Innervation by VII.

Arch 3: stylopharyngeus muscle. lower part of hyoid bone. Innervation by IX.

Arches 4 and 6: muscles of pharynx, larynx and upper esophagus. thyroid and cricoid cartilage. Innervation by X.

B. Pharyngeal Pouches
Be able to identify the pharyngeal pouches in a picture or drawing of an embryo and what things are derived from them.

1st pharyngeal pouch: the tympanic cavity and auditory tube. The endoderm of this pouch will fuse to the ectoderm of the 1st pharyngeal cleft to form the tympanic membrane.

2nd pharyngeal pouch: palatine tonsil.

3rd pharyngeal pouch: inferior parathyroid gland and thymus.

4th pharyngeal pouch: superior parathyroid and the parafollicular (C) cells of the thyroid.



C. Tongue

Develops from swellings on the inner surface of the pharyngeal arches. The pharyngeal arch of origin predicts the sensory innervation of the tongue
- anterior 2/3 from arch 1 = Cn V3
- posterior 1/3 from arch 3 = Cn IX
- root of tongue and epiglottis from arch 4 = Cn X





D. Thyroid
The thyroid gland develops in the ventral midline between the 1st and 2nd pharyngeal arches at the foramen cecum. It is connected to the developing surface of the tongue by the thyroglossal duct.

The thyroid migrates caudally while maintaining its thyroglossal duct connection to the tongue. Over time, the thyroglossal duct degenerates. Remnants of the duct might persist as thyroglossal cysts. These cysts are always in the ventral midline in the neck. In addition, ectopic thyroid tissue might be found anywhere along the developmental migratory path of the thyroid.






E. Ear
- Otic pit > otic vesicle > membranous labyrinth of inner ear.
- pharyngeal arches 1 and 2 > middle ear ossicles
- endodermal pouch 1 > auditory tube




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