Pharynx

Embryology

Lymphoid tissues develop at several sites around the oro- and nasopharynx. In each case the endodermal epithelium proliferates and grows into the surrounding neural crest mesenchyme as solid buds, which form fossae and crypts. Lymphoid cells accumulate around the crypts from the fifth month and become grouped into lymphoid follicles. Four pairs of invaginations form the tubal tonsils, the adenoid (pharyngeal) tonsil, the palatine tonsils and the lingual tonsils: collectively, these accumulations of mucosa associated lymphoid tissue (MALT) are termed the pharyngeal lymphoid ring (Waldeyer's ring).

The embryonic pharynx is broad laterally and compressed dorso­ventrally. From stage 13 (31–33 days post fertilization) the lateral extensions of the pharyngeal pouches with thickening endodermal epithelium can be seen, whereas the pharyngeal roof is still thin. The first pharyngeal pouch gives rise laterally to the tubotympanic recess, which forms the middle ear cavity and the pharyngotympanic tube. The orifice of this tube lies above the palate in the nasopharynx and is the site of the tubal tonsil. The pharyngeal tonsil (adenoid) develops in the roof and posterior wall of the nasopharynx and grows rapidly after birth. The adenoid is large relative to the volume of the nasopharynx at 5 years and undergoes some involution from 8 to 10 years. The second pharyngeal pouch gives rise to the palatine tonsils, which form in the lateral wall of the oropharynx. They grow rapidly in size for the first 5–6 years of life, developing a branching crypt system, and undergo involution at puberty. The lingual tonsils develop on the posterior third of the tongue and form deep crypts in the submucosa.

Most of the pharyngeal content develops from the third, fourth and sixth pharyngeal arches. The third arch gives rise to stylopharyngeus and most of the hyoid bone, and is innervated by the glossopharyngeal nerve. The fourth arch gives rise to the three constrictor muscles and its innervation is based on the superior laryngeal nerve, while most of the laryngeal musculature develops from the sixth arch, which is innervated by the recurrent laryngeal nerve.