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Abstract
Large molecular weight tracers (india ink or albumin labeled with colloidal gold,
Evans blue or rhodamine) were micro-injected into the perivascular space of an artery
or vein on the brain surface, or within the cerebral cortex or the subarachnoid space
of anesthetized rats. The subsequent distribution was followed both under intravital
microscopy, in order to outline the pathways and direction of tracer movement, and
in histological section, in order to describe the pathways of flow at the light and
electron microscopic level. The tracers remained largely in the perivascular spaces
and in the interconnecting network of extracellular channels, including the subpial
space and the core of subarachnoid trabeculae. Tracer also leaked across the pia into
subarachnoid CSF. Bulk flow of fluid within the perivascular space, around both arteries
and veins, was suggested from video-densitometric measurements of fluorescently labeled
albumin. However, this flow was slow, and its direction varied in an unpredictable
way. These results confirm that perivascular spaces may serve as channels for fluid
exchange between brain and CSF, but do not support the idea that CSF circulates rapidly
through brain tissue via perivascular spaces.