Activity of the non-specific alcaline phosphatase appears in brain capillaries in the course of intrauterine development, whereas that of the pseudocholinesterase (butyrylcholinesterase) appears only after birth. This latter enzyme is located in a perinuclear localization at the beginning and occupies later the entire cytoplasm of the endothelial cell. The definitive enzymo-histochemical structure of the capillary is achieved on the 21st day of postnatal development. By means of electron histochemistry, the reaction product of the butyrylcholinesterase reaction appears to be located in and around pinocytotic vesicles and in the perinuclear cysternae of endothelial cells. The results obtained are in harmony with the idea that butyrylcholinesterase is somehow involved in the function of the blood-brain barrier.
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