Comparison of perivascular astrocytic structure between white matter and gray matter of rats. 
Regional differences in perivascular glial structures were investigated between the white matter (hippocampal fimbria, corpus callosum, cerebellar medulla) and the gray matter (cerebral cortex) of adult rats.  Sparser vascular distribution, perivascular glial rows and cylinder-like segmented astrocytic endings were characteristic in the white matter.  The perivascular astrocytic processes covered microvessels extensively in the white matter.  Comparison of transverse microvessel sections by transmission electron microscopy (TEM) between the two sites revealed significantly larger perivascular astrocytic spaces in the white matter.