PER expression in perineurial glia of the dorsal brain. GFP-positive perineurial glia covering brain surface visualized in NP6293-GAL4>GFP flies. (A) Representative brains showing GFP, PER, and combined labeling of the cell nuclei (arrowheads) in young (5 days) and old (55 days) brains at ZT22. At ZT10, PER is absent in perineurial glia from both young old brains. Scale bars equal 10 μm. (B) Brain outline indicating location of cells shown in (A) in the dorsal surface region. (C) Graph showing the average relative fluorescence of PER in perineurial glia. PER levels at ZT22 were significantly lower in old brains (****p ≤ 0.0001, t = 8.928, df = 9.171). Number of brains analyzed are shown within each bar; error bars indicate standard error of the mean (SEM).