Diffusion of horseradish peroxidase perfused through the lateral ventricle of the chick telencephalon.

Horseradish peroxidase, perfused into the lateral ventricle of chick brain, freely and slowly diffuses through the cerebral extracellular spaces. The layer of astrocytic end-feet surrounding blood capillaries does not consitute a barrier to the tracer which permeates the basal lamina, diffuses between the pericytic cells and finally accumulates in the intercellular space beneath the tight junctions between contiguous endothelial cells. No evidence was found for transport by micropinocytotic vesicles from the cerebral parenchyma to the capillary lumen.