Ions were allowed to diffuse through the diaphragms of mice. They were then precipitated by other ions added to the fixative. The precipitates were found in the intercellular junctions of the mesothelium, the lymphatics and the blood capillaries. They appeared to traverse zonulæ occludentes quite easily. They were also found apparently traversing the cells in small vesicles. It is considered that the passage through the junctions is quantitatively much more important than that via the vesicles. Molecules of m. wt. 〈1,000 can readily pass through the junctions, but those〉 20,000 can not do so. It is therefore concluded that the ‘meshes’ of the zonulæ occludentes admit molecules up to a diameter of ∼2–4 mµ. This is in good agreement with Pappenheimer's (1953) ‘slits’ with a width of ∼4 mµ. It is known that these would have an area approximately equal to that of the intercellular junctions. Lipid‐soluble substances may also traverse the endothelium by dissolving in the plasma membranes, and so pass through or around the cells.
No takes yet. Share an insight, caveat, or question.
J. R. Casley‐Smith (1967) studied this question.