The Ecto-Endodermal Boundary in the Oral and Pharyngeal Mucosa: A Narrative Review.

Introduction: The ecto-endodermal boundary in the human oral cavity remains debated. Rather than a distinct line, this transition forms a complex interface, especially in the developing mouth and pharynx. This ambiguity is clinically relevant, as, for example, HPV-induced tumors often arise where ecto- and endoderm meet. The boundary is generally placed at the posterior third of the tongue and behind the uvula, though its developmental basis remains unclear. This review summarizes the literature on the embryological development of the mouth and pharynx to trace origins and interactions of ectodermal and endodermal derivatives and identify potential tumor initiation regions. Methods: PubMed articles on oral structures were reviewed to approximate the ecto-endodermal border. Results: Teeth originate from ectoderm. The origin of salivary glands and papillae depends on their location. No study reports were found for the tonsils, incisive papilla, tubarial glands, and faucial pillars. Conclusions: A clear educational image of the origin of the structures of the human mouth is made to help identify regions of risk for oral cancer.