Enzyme cytochemistry of the cerebral microvessel wall.

The cytochemical localisation of five hydrolytic enzymes has been studied in the brain capillaries of laboratory animals. Acid phosphatase is present in primary lysosomes of endothelial cells; alkaline phosphatase activity is seen mainly on the plasma membrane of the luminal side but also in the basal lamina. The latter is also active concerning 5'nucleotidase. Butyrylcholinesterase is an enzyme synthesized by most brain capillary endothelial cells, as can be seen by intensive staining of endoplasmic reticulum cisternae. In contrast acetylcholinesterase activity at the capillaries presumably is of neuronal origin. Local neurons appear to secrete this enzyme, which then reaches the endothelial basal lamina via the extracellular spaces. From these cytochemical observations it is concluded that pinocytotic traffic in brain endothelial cells is predominantly from the brain tissue side to the luminal side.