Figure 1: Quercetin effectively alleviates the retinal histopathological alterations induced by STZ in diabetic rats. (A) Histopathological observations of retinas (hematoxylin and eosin staining, original magnification 40×, scale bars: 50 μm). Normal control group exhibited normal thickness of retinas as well as an ordered arrangement of retinal cells with a uniform distribution. Rats in the STZ-treated group showed pathological features of DR at 16 weeks after STZ-induced diabetes, including marked reductions in the thickness of the INL and ONL, the distance between the ILM and OLM, as well as in the number of ganglion cells. These retinal changes were partially ameliorated by treatment with quercetin. Downregulating HO-1 expression by Znpp partially reversed the neuroprotective effect of quercetin in DR. (B) Quantitative analysis of the distance between the ILM and OLM (marked by blue arrow in A). (C) Quantitative analysis of the thickness of the INL (marked by yellow arrow in A). (D) Quantitative analysis of the thickness of the ONL (marked by red arrow in A). (E) Quantitative analysis of the number of cells in the GCL. (F) Blood glucose levels. (G) TUNEL staining of retinal sections (original magnification 40×, scale bars: 50 μm). The STZ-treated group showed more TUNEL-positive cells compared with the control group. Treatment with quercetin significantly decreased the percentage of apoptotic cells, and this effect was weakened by Znpp. (H) Quantitative analysis of total cell numbers in the whole section. (I) Quantitative analysis of the percentage of TUNEL-positive cells in the whole section. Data are expressed as the mean ± SD (n = 10). *P < 0.05, **P < 0.01 (one-way analysis of variance followed by the Kruskal-Wallis test). GCL: Ganglion cell layer; ILM: inner limiting membrane; ILN: inner nuclear layer; NS: not significant; OLM: outer limiting membrane; ONL: outer nuclear layer; Q: quercetin; STZ: streptozocin; TUNEL: terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling; Znpp: zinc protoporphyrin IX.