Mucous metaplasia and gap junctions in the vitamin A acid-treated skin tumor, keratoacanthoma.

Desmosomes are the usual cell junctions found in normal rabbit epithelium as well as in the untreated keratoacanthoma. This study reports the finding of a second cell junction, the gap junction, when epithelium, normal or tumorous, is subjected to topical applications of vitamin A acid. The gap junction forms early in mucous metaplasia (after 2 days of application of vitamin A acid) and appears before the gross appearance of mucus. The presence of the gap junction occurs when there is an increase in the rough-surfaced endoplasmic reticulum and Golgi cisternae and vesicles. It is possible that the early appearance of the gap junction facilitates and mediates the mucous metaplasia. This suggestion is strengthened by the fact that the gap junction, once present in the mucus-producing tumor, is sparse when the tumor reverts back to the dry, keratotic condition upon cessation of vitamin A acid applications.