Histological appearance of the small intestine of wild type and CF mice on the different genetic backgrounds. Tissue was paraffin embedded and stained with PAS for neutral mucins. The sections are from the middle portion of the small intestine (A) Wild type on the B6 background. (B) CF on the B6 background. (C) Wild type on the mixed background. (D-F) CF on the mixed background. (A, C) In the wild type tissue from both backgrounds, the crypts are small and have narrow lumina (arrows). (B) In the CF tissue on the B6 background, the crypt lumina are greatly dilated and filled with PAS-reactive mucus (arrows). In some CF mice on the mixed background, the crypts are not apparently different than wild type (D), and the crypts are normal appearing (arrows). Some CF mice on the mixed background had mildly affected crypts (E), while others had greater crypt dilation and mucus accumulation (F). Overall, the CF mice on the mixed background were less severely affected compared to those on the B6 background.