Opening of tight junctions in frog skin by hypertonic urea solutions

The movements and distribution of lanthanum have been studied in frog skins treated on the outside with Ringer's solution to which 400MM urea had been added. Under these conditions,(140)La moved across the skin. This finding is in contrast with what is observed in control skins, which are impermeable to lanthanum. Examination with the electron-microscope of skins fixed during equilibration with outside hypertonic solutions plus lanthanum, showed penetration of the tracer from the outside into the intercellular spaces of all the inner layers of the epidermis. The occluding zonules (tight junctions) that normally prevent the inward movement of lanthanum beyond the outer border of the stratum granulosum (SG) are open. As a consequence, most intercellular spaces along the lateral cell surfaces of the SG were permeated by lanthanum throughout their extension. In skins treated with hypertonic solutions and then allowed to recover in normal Ringer's, the occluding zonules of the SG were impermeable to lanthanum, and(140)La did not move across the epidermis. These observations show that the increased skin permeability caused by outside hypertonic solutions results mainly from a reversible opening of tight junctions.