Internalized gap junctions in ciliary epithelium of rabbit and rat. A transmission electron-microscopic study.

Transmission electron microscopy was used to examine internalized gap junctions (IGJ) in rabbit and rat ciliary epithelial cells. A prominent feature of all the specimens studied was the presence of different images of IGJ membrane that entrapped a portion of an adjoining cell. We documented and analyzed more than 500 gap junction (GJ) vacuoles and invaginations, the latter comprising less than 20% of all the structures examined. With ten exceptions found in non-pigmented cells, all the IGJ were unidirectionally internalized within the cytoplasm of pigmented epithelial cells. Morphological signs of autophagic degradation of GJ vacuoles were observed. An essential finding was that once a GJ membrane started to invaginate, a "lucidation" of a part of the protruding cytoplasm occurred; no planar GJ membranes exhibited such an alteration. The present finding suggest that IGJ derived from the epithelium of ciliary processes arise through an invagination-endocytosis mechanism and are degraded autophagically. This phenomenon may be relevant to aqueous humor production.