Cytochemical Study of Alkaline Phosphatase Activity in Microvessels of Rat Cerebral Cortex

Intracortical microvessels of the rat frontal lobe were examined by light and electron microscopy with special reference to the relationship between the localization of alkaline phosphatase activity on the walls and the luminal diameters of the vessels. The lead citrate method of Mayahara et al.2) was used to histochemically demonstrate alkaline phosphatase. At the electron microscopic level, electron dense reaction products of alkaline phosphatase activity were found on the walls of arterioles with one medial layer and in those of metarterioles with single-layered and incomplete media. However, no reaction was encountered on the walls of arterioles with 2-3 medial layers. On the walls of capillaries 6-9 μm in luminal diameter, the reaction products of alkaline phosphatase were distributed discontinuously on the abluminal surfaces of endothelial cells and on the surfaces of pericytes. Intense reaction products were distributed evenly on the surfaces of astrocytic processes in contact with the thick basal lamina. On the other hand, capillaries less than 5 μm in luminal diameter showed no reaction at all. The walls of venules and veins also displayed no activity for this enzyme. In view of increasing interest in the physiology of cerebral microcirculation and the blood-brain barrier, the ultrastructural localization of alkaline phosphatase would cbntribute to the understanding of this system.