Respiratory epithelium in an apical periodontal cyst of the mandible.

A cyst attached to the apex of a mandibular canine was partly lined with respiratory epithelium. This is an interesting observation for two reasons: (1) The few well-documented instances of this type of epithelium in apical periodontal cysts have been in the maxilla, where a plausible source for the respiratory epithelium for most of them was a breach in the floor of the maxillary sinus. (2) The mandibular location of the cyst reported here indicates that the respiratory epithelium within it was derived through proliferation and metaplasia of the rests of Malassez.