Accumulation of Protein Tracers in Pericytes of the Central Nervous System Following Systemic Injection in Immature Mice

Following systemic injection of protein tracers (albumin labelled with Evan's blue and horseradish peroxidase) in immature mice, cells along cerebral and spinal cord vessels accumulated significant amounts of these proteins as revealed by fluorescence microscopy and enzyme histochemistry. These cells most probably correspond to pericytes which in adult animals contain lipofuscin granules and lysosomal enzymes. This accumulation could be revealed only in animals taken 24 h or later after the injection. No tracers were seen in the surrounding neuropile, in glial cells or in neurons. Our observation must imply (1) that small amounts of systematically injected protein tracers can actually pass from the blood into the brain in immature mice and (2) that pericytes of the normal immature brain have the capacity to engulf such tracers from their immediate surroundings.