Splenic atrophy and fibrosis were associated with functional hyposplenism in aging HO-1^−/− mice. (A) In mice younger than 9 months, the spleen to brain weight ratio of HO-1^−/− mice (n = 6) was 2.3-fold (P < .001) higher than in WT (n = 6), whereas HO-1^−/− mice older than 9 months (n = 6) had on average 1.6-fold (P < .05) smaller spleens than their WT counterparts (n = 4). Error bars represent SD of the mean. Graphing of individual spleen/brain ratios (right panel) indicated that splenic swelling was notable in young HO-1^−/− mice, whereas older mice had progressively more severe atrophy. (B) Progressive fibrosis developed in the red pulp of aging HO-1^−/− mice as observed by Masson trichrome histochemistry. Arrowheads point to collagen fibers, which appear as blue streaks. (C) Peripheral blood smears of HO-1^−/− mice showed increased numbers of abnormally shaped RBCs (arrowheads) and numerous cells with Howell-Jolly bodies (arrows). Four animals from each group were examined. One typical sample of WT and 2 of HO-1^−/− (14 months old) are shown.