This paper reports on the discovery of hexagonally packed subunits in intercellular junctions of the mouse heart and liver using a novel technique involving lanthanum salts. The authors, J. P. Revel and M. J. Karnovsky, describe a method where tissue blocks are treated with lanthanum salts before dehydration and embedding, which fills the extracellular space with an electron-opaque material. This material clearly delineates hexagonal structures in specialized intercellular junctions, particularly in the heart and liver. The subunits have an electron-opaque core and a thin electron-transparent wall, with a center-to-center spacing of about 90 Å. The study also confirms the presence of tight junctions and cell junctions with a minute gap between the external leaflets, where lanthanum can permeate. The findings suggest that the hexagonal pattern may be characteristic of junctions involved in electrical interconnection between cells. The work was supported by grants from the National Institutes of Health and the United States Public Health Service.This paper reports on the discovery of hexagonally packed subunits in intercellular junctions of the mouse heart and liver using a novel technique involving lanthanum salts. The authors, J. P. Revel and M. J. Karnovsky, describe a method where tissue blocks are treated with lanthanum salts before dehydration and embedding, which fills the extracellular space with an electron-opaque material. This material clearly delineates hexagonal structures in specialized intercellular junctions, particularly in the heart and liver. The subunits have an electron-opaque core and a thin electron-transparent wall, with a center-to-center spacing of about 90 Å. The study also confirms the presence of tight junctions and cell junctions with a minute gap between the external leaflets, where lanthanum can permeate. The findings suggest that the hexagonal pattern may be characteristic of junctions involved in electrical interconnection between cells. The work was supported by grants from the National Institutes of Health and the United States Public Health Service.