This study reports the discovery of hexagonally packed subunits in intercellular junctions of the mouse heart and liver using a new technique involving lanthanum salts. The technique involves treating tissue blocks with lanthanum nitrate before dehydration and embedding, which fills the extracellular space with an electron-opaque material, allowing the visualization of hexagonal structures in specialized junctions. The results show that these structures are present in both heart and liver tissues, with similar patterns observed in previously studied liver plasma membrane fractions. The hexagonal structures are clearly delineated in the extracellular space, particularly in the intercalated discs of the heart and in junctional areas of the liver. The structures appear to be accessible to small colloidal molecules, suggesting they are permeable to lanthanum. In the heart, the structures are outlined in the extracellular space, including collagen fibrils and other cellular components. In the liver, similar hexagonal packing is observed in junctional areas between the space of Disse and bile capillaries. The study also shows that these structures are similar to those previously observed in electrical synapses of Mauthner cells. The hexagonal packing is confirmed by optical diffraction and print rotation techniques. The structures are interpreted as hollow prisms with a hexagonal arrangement, with a core and a wall. The study concludes that lanthanum impregnation is useful for tracing the extracellular space in electron microscope specimens. The findings suggest that hexagonal structures may be characteristic of some, but not all, junctions involved in electrical interconnection between cells. The study was supported by grants from the National Institutes of Health.This study reports the discovery of hexagonally packed subunits in intercellular junctions of the mouse heart and liver using a new technique involving lanthanum salts. The technique involves treating tissue blocks with lanthanum nitrate before dehydration and embedding, which fills the extracellular space with an electron-opaque material, allowing the visualization of hexagonal structures in specialized junctions. The results show that these structures are present in both heart and liver tissues, with similar patterns observed in previously studied liver plasma membrane fractions. The hexagonal structures are clearly delineated in the extracellular space, particularly in the intercalated discs of the heart and in junctional areas of the liver. The structures appear to be accessible to small colloidal molecules, suggesting they are permeable to lanthanum. In the heart, the structures are outlined in the extracellular space, including collagen fibrils and other cellular components. In the liver, similar hexagonal packing is observed in junctional areas between the space of Disse and bile capillaries. The study also shows that these structures are similar to those previously observed in electrical synapses of Mauthner cells. The hexagonal packing is confirmed by optical diffraction and print rotation techniques. The structures are interpreted as hollow prisms with a hexagonal arrangement, with a core and a wall. The study concludes that lanthanum impregnation is useful for tracing the extracellular space in electron microscope specimens. The findings suggest that hexagonal structures may be characteristic of some, but not all, junctions involved in electrical interconnection between cells. The study was supported by grants from the National Institutes of Health.