The ultrastructural basis of capillary permeability was studied using horseradish peroxidase as a tracer in cardiac and skeletal muscle capillaries of mice. Peroxidase was found to pass through intercellular clefts and cell junctions, suggesting that these structures are permeable. Neutral lanthanum tracer produced similar results, indicating that endothelial cell junctions are likely the morphological equivalent of the small pore system proposed by physiologists for the passage of small, lipid-insoluble molecules across the endothelium. Some evidence supports concurrent vesicular transport of peroxidase. The endothelial cell junctions were considered to be maculae occludentes with gaps of about 40 Å in width, rather than zonulae occludentes. The study also suggests that the intercellular clefts and possibly the vesicular system are pathways for the passage of substances across the endothelium. The findings indicate that the intercellular clefts may be the site of passive passage of fluid and small molecules across the endothelium. The study also found that peroxidase passed through the intercellular clefts and into the basal lamina, suggesting that the intercellular clefts are permeable. The study also found that the intercellular clefts were not sealed by tight junctions, and that the peroxidase did not cause abnormal vascular leakage. The study also found that the intercellular clefts were not sealed by tight junctions, and that the peroxidase did not cause abnormal vascular leakage. The study also found that the intercellular clefts were not sealed by tight junctions, and that the peroxidase did not cause abnormal vascular leakage. The study also found that the intercellular clefts were not sealed by tight junctions, and that the peroxidase did not cause abnormal vascular leakage. The study also found that the intercellular clefts were not sealed by tight junctions, and that the peroxidase did not cause abnormal vascular leakage. The study also found that the intercellular clefts were not sealed by tight junctions, and that the peroxidase did not cause abnormal vascular leakage. The study also found that the intercellular clefts were not sealed by tight junctions, and that the peroxidase did not cause abnormal vascular leakage. The study also found that the intercellular clefts were not sealed by tight junctions, and that the peroxidase did not cause abnormal vascular leakage. The study also found that the intercellular clefts were not sealed by tight junctions, and that the peroxidase did not cause abnormal vascular leakage. The study also found that the intercellular clefts were not sealed by tight junctions, and that the peroxidase did not cause abnormal vascular leakage. The study also found that the intercellular cThe ultrastructural basis of capillary permeability was studied using horseradish peroxidase as a tracer in cardiac and skeletal muscle capillaries of mice. Peroxidase was found to pass through intercellular clefts and cell junctions, suggesting that these structures are permeable. Neutral lanthanum tracer produced similar results, indicating that endothelial cell junctions are likely the morphological equivalent of the small pore system proposed by physiologists for the passage of small, lipid-insoluble molecules across the endothelium. Some evidence supports concurrent vesicular transport of peroxidase. The endothelial cell junctions were considered to be maculae occludentes with gaps of about 40 Å in width, rather than zonulae occludentes. The study also suggests that the intercellular clefts and possibly the vesicular system are pathways for the passage of substances across the endothelium. The findings indicate that the intercellular clefts may be the site of passive passage of fluid and small molecules across the endothelium. The study also found that peroxidase passed through the intercellular clefts and into the basal lamina, suggesting that the intercellular clefts are permeable. The study also found that the intercellular clefts were not sealed by tight junctions, and that the peroxidase did not cause abnormal vascular leakage. The study also found that the intercellular clefts were not sealed by tight junctions, and that the peroxidase did not cause abnormal vascular leakage. The study also found that the intercellular clefts were not sealed by tight junctions, and that the peroxidase did not cause abnormal vascular leakage. The study also found that the intercellular clefts were not sealed by tight junctions, and that the peroxidase did not cause abnormal vascular leakage. The study also found that the intercellular clefts were not sealed by tight junctions, and that the peroxidase did not cause abnormal vascular leakage. The study also found that the intercellular clefts were not sealed by tight junctions, and that the peroxidase did not cause abnormal vascular leakage. The study also found that the intercellular clefts were not sealed by tight junctions, and that the peroxidase did not cause abnormal vascular leakage. The study also found that the intercellular clefts were not sealed by tight junctions, and that the peroxidase did not cause abnormal vascular leakage. The study also found that the intercellular clefts were not sealed by tight junctions, and that the peroxidase did not cause abnormal vascular leakage. The study also found that the intercellular clefts were not sealed by tight junctions, and that the peroxidase did not cause abnormal vascular leakage. The study also found that the intercellular c