The epithelia of various glands and cavitary organs in rats and guinea pigs were examined, revealing a characteristic tripartite junctional complex between adjacent cells. This complex, consisting of zonula occludens (tight junction), zonula adhaerens (intermediate junction), and macula adhaerens (desmosome), is found in mucosal epithelia of the stomach, intestine, gall bladder, uterus, and oviduct; glandular epithelia of the liver, pancreas, parotid, stomach, and thyroid; and in epithelia of pancreatic, hepatic, and salivary ducts. It is also present between epithelial cells of the nephron (proximal and distal convolutions, collecting ducts). The elements of the complex are located in a juxtaluminal position and occur in the order of tight junction, intermediate junction, and desmosome in an apical-basal direction. The tight junction is characterized by the fusion of adjacent cell membranes, resulting in the obliteration of the intercellular space over variable distances. A dense outer leaflet of the adjoining cell membranes converges to form a single intermediate line. A diffuse band of dense cytoplasmic material is often associated with this junction, though its development varies between epithelia. The intermediate junction is characterized by an intercellular space (approximately 200 Å) occupied by homogeneous, apparently amorphous material of low density. The adjoining cell membranes are strictly parallel over distances of 0.2 to 0.5 μ, and there are conspicuous bands of dense material in the subjacent cytoplasmic matrix. The desmosome is characterized by an intercellular space (approximately 240 Å) containing a central disc of dense material. Discrete cytoplasmic plaques are disposed parallel to the inner leaflet of each cell membrane, and bundles of cytoplasmic fibrils converge on the plaques. The tight junction forms a continuous belt-like attachment, while the desmosome is a discontinuous, button-like structure. The intermediate junction is continuous in most epithelia but discontinuous in some. Observations during experimental hemoglobinuria showed that hemoglobin, which undergoes concentration in the nephron lumina to act as an electron-opaque mass tracer, does not penetrate the intercellular spaces beyond the tight junction. Similar observations were made in pancreatic acini and ducts where discharged zymogen served as a mass tracer. Hence, the tight junction is impervious to concentrated protein solutions and functions as a diffusion barrier or "seal." Specialized intercellular junctions, known as desmosomes and terminal bars, have been studied extensively by electron microscopy. These junctions are recognized as local modifications of the surface of adjacent yet separate cells, rather than intercellular bridges. Their functional significance includes their role in cell-to-cell adhesion andThe epithelia of various glands and cavitary organs in rats and guinea pigs were examined, revealing a characteristic tripartite junctional complex between adjacent cells. This complex, consisting of zonula occludens (tight junction), zonula adhaerens (intermediate junction), and macula adhaerens (desmosome), is found in mucosal epithelia of the stomach, intestine, gall bladder, uterus, and oviduct; glandular epithelia of the liver, pancreas, parotid, stomach, and thyroid; and in epithelia of pancreatic, hepatic, and salivary ducts. It is also present between epithelial cells of the nephron (proximal and distal convolutions, collecting ducts). The elements of the complex are located in a juxtaluminal position and occur in the order of tight junction, intermediate junction, and desmosome in an apical-basal direction. The tight junction is characterized by the fusion of adjacent cell membranes, resulting in the obliteration of the intercellular space over variable distances. A dense outer leaflet of the adjoining cell membranes converges to form a single intermediate line. A diffuse band of dense cytoplasmic material is often associated with this junction, though its development varies between epithelia. The intermediate junction is characterized by an intercellular space (approximately 200 Å) occupied by homogeneous, apparently amorphous material of low density. The adjoining cell membranes are strictly parallel over distances of 0.2 to 0.5 μ, and there are conspicuous bands of dense material in the subjacent cytoplasmic matrix. The desmosome is characterized by an intercellular space (approximately 240 Å) containing a central disc of dense material. Discrete cytoplasmic plaques are disposed parallel to the inner leaflet of each cell membrane, and bundles of cytoplasmic fibrils converge on the plaques. The tight junction forms a continuous belt-like attachment, while the desmosome is a discontinuous, button-like structure. The intermediate junction is continuous in most epithelia but discontinuous in some. Observations during experimental hemoglobinuria showed that hemoglobin, which undergoes concentration in the nephron lumina to act as an electron-opaque mass tracer, does not penetrate the intercellular spaces beyond the tight junction. Similar observations were made in pancreatic acini and ducts where discharged zymogen served as a mass tracer. Hence, the tight junction is impervious to concentrated protein solutions and functions as a diffusion barrier or "seal." Specialized intercellular junctions, known as desmosomes and terminal bars, have been studied extensively by electron microscopy. These junctions are recognized as local modifications of the surface of adjacent yet separate cells, rather than intercellular bridges. Their functional significance includes their role in cell-to-cell adhesion and