Parenchymal and perivascular collagen-containing fibrils accumulate selectively in ERK2 cKO frontal cortex. A: Gallyas silver degeneration stain reveals numerous wavy parenchymal fibrils in cKO frontal cortex (B) but not in controls (A). Aberrant fibrils are abundant in the frontoparietal cortex (C) of a 6-week-old ERK2 cKO mice and completely absent from occipital cortex (D) of the same animal. Reticulin staining (E–H) reveals, in addition to the isolated parenchymal fibrils highlighted by Gallyas stain, abundant pericapillary matrix material in cKO cortex (F and H). Wild-type (wt) animals show reticulin-positive matrix material restricted to large penetrating arteries/arterioles in superficial cortex (arrow, E). Scale bars represent 50 μm (A, B, G, and H), 100 μm (C–F), and 1 μm (G).