The Ultrastructure of the Nexus

A correlation is made between the appearances of the nexus ("gap junction") as revealed by thin-section and by freeze-cleave electron microscopy techniques. These methods reveal different aspects of a complex subunit assembly forming the nexus membranes. In thin sections, the nexus is formed by the very close apposition of two "unit" membranes. The electron-opaque tracer, colloidal lanthanum hydroxide, outlines an aspect of electron-lucent subunits that project into the central region of the nexus. The freeze-cleave technique demonstrates novel membrane faces that are generated from within the interior of plasma membranes by splitting them into two lamellae (Lm): Lm 1 adjacent to the cytoplasm, and Lm 2 adjacent to the extracellular space. Each of the two membranes forming the nexus can be split into these two lamellae. On the new face of Lm 1, particles approximately 50 A in diameter are closely packed in an array which is often hexagonal with a 90-100 A center-to-center spacing. The two apposed lamellae (Lm 2-Lm 2) of the nexus are constructed of sheets of subunits in a similar array. The Lm 1 particles appear to extend into the Lm 2 subunits to form macromolecular complexes. The Lm 2 subunits extend to the center of the nexus to form the contacts outlined by lanthanum in sections. It is postulated that central hydrophilic channels may extend through the subunit assembly to provide a direct route for intercellular communication.