Electron microscopic observations on the vascular barrier in the cortex of the thymus of the mouse

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Abstract
The thymus of newborn and young adult mice of the DBA strain was studied under the electron microscope after fixation in osmium tetroxide and embedding in araldite. In most of the animals several intravenous doses of Thorotrast were administered within 24 or 48 hours of thymectomy.The fine vessels in the cortex of the thymus appear unusually competent. Their endothelium is complete, their basement membrane broad, and their wall further increased by adventitial cells and extracellular tissue. The epithelial reticular cell is the most peripheral adventitial element of these vessels. It extends slender processes which encircle the vessel and, by similar processes, surrounds individual or clusters of perivascular lymphocytes. This cell is disposed to mediate influences passing between cortical lymphocytes and the blood.By cytoplasmic processes of the endothelium and the adventitial reticular cells a cellular pathway from the lumen to the outside of the these vessels is established. In smaller vessels, moreover, the adventitial enclosure by reticular cells is incomplete.After intravenous injection thorium dioxide may be found in the vessel wall and, to a limited degree, in the surrounding tissue.These vessels resemble both the fine vessels in the central nervous system where a blood‐brain barrier is present, and the terminal arterial vessels in the spleen.
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