Lanthanium hydroxide tracer studies on rat corneal endothelum

Abstract

The ultrastructure of corneal endothelium of rats was examined using two aldehyde fixatives, including glutaraldehyde. In addition, the lanthanum-hydroxide method was employed to trace spaces continuous with the intercellular space. The extracellular gap between the endothelial cells varied from about 250–400 Å. In the junctional complexes beneath the endothelial surface, a closer apposition of cell membranes, formerly described as terminal bar or tight junction, can be observed. The intercellular space is here reduced to about 30 Å, but remains still wider than the one normally observed in tight junctions. “En face” sections of cell junctions showed evidence of subunits characteristic of nexus (“gap junction”). Colloidal lanthanum-hydroxide, used as an electron-opaque marker, penetrated readily through these “gap junctions” and was found as accumulations of electron-dense granules on the innermost layers of Descemet's membrane. This penetration of an electron-opaque marker of about 20 Å minimal diameter along the intercellular space into Descemet's membrane in the absence of pinocytotic activity of endothelial cells lends support to the hypothesis, that the formerly described “tight junctions” (zonulae occludentes or terminal bars) between endothelial cells are to be considered as “gap junctions” with gaps of approximately 30 Å between two apposed cell membranes. The penetration of ions and larger molecules as represented by the lanthanum-hydroxide across the junctional complexes seems to be possible even without the formerly postulated mechanisms of a partly intracellular transport by means of pinocytosis.