Localization of alkaline phosphatase activity in endothelia of developing and mature mouse blood-brain barrier

The relationship between alkaline phosphatase (AP) activity and maturation of blood-brain barrier (BBB) in mouse brain was studied ultracytochemically. The permeability of micro-blood vessels (MBVs) to intravenously injected horseradish peroxidase was considered to be a criterion of BBB maturation, which occurs between the 12th and 24th day of mouse life. This process coincides with the appearance of cytochemically detectable AP activity in luminal plasma membrane (PM) of the endothelial cell (EC) of capillaries or in both luminal and abluminal PM in arterioles. No reaction appears in ECs of venules. These observations indicate that the development of BBB function is accompanied by the formation of enzymatic barrier in MBVs endothelia. AP constitutes one of the components of this barrier. Comparative study indicates that in various MBVs of non-BBB type, except in skeletal muscle, no AP activity is detected in similar incubation conditions.