Specialized Localization of P‐Glycoprotein Recognized by MRK 16 Monoclonal Antibody in Endothelial Cells of the Brain and the Spinal Cord

This communication describes the cellular and ultrastructural localization in the central nervous system of P‐glycoprotein (P‐GP) recognized by a murine monoclonal antibody, MRK 16. At the ultrastructural level P‐GP was strictly confined to the luminal surface of the endothelial cells which comprise the capillary vessels of the brain and the spinal cord. No P‐GP was found in the endothelial cells of other organs. Our findings may be useful as a means to define the blood‐brain barrier, and they imply that the blood‐brain barrier is anatomically characterized by the presence of intercellular tight junctions between continuous nonfenestrated endothelial cells.