Ultracytochemical evidence for endothelial channel-lysosome connections in mouse brain following blood-brain barrier changes.

An investigation designed to define relationships between endothelial channels and lysosomes was conducted in the mammalian brain microvasculature. Microvessels from normal and mechanically injured mouse brains were studied ultracytochemically for: (1) transport of horseradish peroxidase (HRP) protein tracer through endothelial channels, and (2) for acid phosphatase (AcP) activity as an enzymatic marker of lysosomes. Following traumatic brain injury for 1 week with 2 h circulation of intravenously injected HRP, selected brain slices were processed for ultrastructural localization of either HRP, AcP, or for both reactions together within the same tissue slices. One week after blood-brain barrier (BBB) damage, the presence of HRP reaction product (RP) was observed within endothelial channels and vesicles of capillaries and arterioles with concomitant increase in lysosomal enzymatic activity of the endothelial cells bordering regions of brain damage. Lysosomes were observed to be directly connected to the endothelial channels. Our observations present cytochemical evidence for endothelial channel-lysosome connections which may suggest intralysosomal modification of blood-born materials before entering the neuropil. Such modification could have important immunological and/or metabolic significance.