Diminished insulin stores and mildly disorganized islets in ERO1-β mutant mice. (A) Hematoxylin- and eosin-stained pancreatic sections of 3-mo-old wild-type and homozygous ERO1-β mutant mice. (B) Pancreatic insulin content of mice as in A. Shown are the mean and SEM (n = 5; *, P < 0.001 by two-tailed t test). (C) Fluorescent micrographs of frozen section of pancreas from wild-type (+/+) and ERO1-β homozygous mutant mice (i/i; as in A) stained with antisera to insulin (Ins), glucagon, and the karyophilic dye DAPI. The bottom right panel in each genotype is an overlay of the three stains. Note the presence of glucagon-positive cells in the interior of the islet of the ERO1-β mutant. (D) Electron micrographs of insulin-producing β cells from islets of Langerhans of wild-type and ERO1-β mutant mice. Note the lack of ER dilation in the higher magnification views (right) of the mutant sample. Bars: (A and C) 100 µm; (D, left) 2 µm; (D, right) 0.5 µm.