On tight junction structure: Forssman glycolipid does not flow between MDCK cells in an intact epithelial monolayer.

One model of tight junction structure suggests that lipids might flow from cell to cell within shared exoplasmic membrane leaflets. We tested this proposal by co-culturing two clones of MDCK epithelial cells, which differed in their content of Forssman glycolipid, and then staining by immunofluorescence with rabbit anti-Forssman Ig. In co-cultures grown on glass cover slips and on nitrocellulose filters, positive Forssman staining was restricted to sharply demarcated clusters of cells formed by the Forssman-positive clone. Integrity of tight junctions between the two clones was indicated on cover slips by the presence of individual domes (hemicysts) composed of both clones and on filters by the generation of transepithelial potential differences. These results suggest that glycolipids in the exoplasmic leaflet of cells in a tight epithelium do not flow to adjacent cells.