Fig. 2: The epipia attenuates and loosens around small-to-medium leptomeningeal arteries, forming the epipial space.

Schematic (a) and routine sections (b–d) demonstrate the relationships of pial cells on leptomeningeal, i.e., subarachnoid space (SAS) arteries and penetrating arterioles. Medium-sized SAS arteries at the brain surface demonstrate attenuation and loosening of the epipia layer, creating an epipial space (e), whereas smaller SAS arteries and penetrating arterioles demonstrate further thinning and coalescence of pial layers with the vessel walls, with occasional envelopment of the arteriolar smooth muscle cell layer (f, g). Analysis of epipial sheath thickness, expressed as a percentage (%) of vessel area, is shown according to vessel size (h) and brain region (i); Pearson correlation coefficient; n = 86 vessels. Likewise, analysis of epipial fenestration, expressed as a percentage (%) of total vessels, is shown according to brain region (j) and vessel size (k); one-way ANOVA with Tukey’s multiple comparisons test; n = 122 vessels. A medium-sized artery in ventral mouse brain (l, the same vessel shown in e), is depicted at higher magnification and demonstrates the loosened epipial sleeve composed of interlinking pial cells that partially enclose the epipial space (asterisks). Features of the epipial space are highlighted by immunohistochemistry (l, right-hand side), and are shown to advantage in ultrastructural images (m). Enlargements of boxed micrograph areas demonstrate the intra-adventitial space, in which scattered collagen fibrils are appreciated (arrows). Analysis of epipial space areas are shown relative to vessel size (n); Pearson correlation coefficient; n = 115 vessels from 6 mice. Analysis of epipial space areas are shown relative to brain region (o); one-way ANOVA with Tukey’s multiple comparisons test; n = 115 vessels from 6 mice. (b–d, L left) H&E; (e–g, L right) red/CY3, SMA; green/FITC, ERTR7; blue, DAPI; m transmission electron micrographs, with scale bars as indicated (asterisks represent the epipial space); scale bars = (b) 100 μm; c–e 20 μm; f, g 10 μm. Source data are provided as a Source Data file. TEM data are representative of 50 vessels from 3 mice.