Fig. 1: Pial cells are reticulated, express ERTR7, and prominently ensheath large arteries in ventral brain regions, forming the epipia.

Routine (a), ultrastructural (b, c), and super-resolution fluorescent ERTR7 labeled (d) images of the brain surface demonstrate the reticulated nature of pial cells. On coronal section (e), the proximal middle cerebral artery is observed to be thickly ensheathed (open arrow), while a distal oblique arterial segment exhibits a thin sheath (solid arrow) and small arterial branches are unsheathed by ERTR7. Large subarachnoid arteries in ventral brain region are depicted on longitudinal (e) and cross (e, f) sections. Enlargement of the boxed area in f highlights ERTR7-positive tunica adventitia in apposition to the arterial tunica media (g); insets at upper right are representative of the boxed area and show punctate ERTR7 label, consistent with matted pial cell processes. Partial pial ensheathment is appreciated on cross section of a small artery (h), and enlargement of the boxed area (i). Diagram depicting brain regions used for quantification (j). Analysis of epipial coverage according to anatomical brain region (k) and vessel size (l, m); n = 145 vessels from 6 mice. a H&E; b transmission electron micrograph; c Immuno-EM for ERTR7; d–i green/FITC, ERTR7; red/CY3, SMA; blue, DAPI; scale bars = (a–d) 2 μm; c and d (insets) 200 nm; e, f 50 μm; g–i 10 μm. Primary (1^o), secondary (2^o), and tertiary (3^o) processes are depicted within pial cells (b, d). Source data are provided as a Source Data file. TEM data are representative of 50 vessels from 3 young mice and immuno-EM data are representative of 6 sections from 2 young mice.