High TBI doses increase the severity of early GI tract GVHD. Bowel histology in allografted mice treated with 900 cGy TBI (a and c) and 1,300 cGy TBI (b and d). The small bowel of the 900 cGy mouse (a) exhibits mild villous blunting and crypt regenerative changes; crypts in the large bowel (c) exhibit mitotic figures and a rare apoptotic cell (arrowhead). In contrast, small bowel of the 1,300 cGy mouse (b) exhibits more marked villous blunting and crypt regeneration with mitotic figures. The large bowel (d) exhibits crypt atrophy, abundant apoptotic cells (arrowhead), and marked depletion of surface epithelial mucous. Original magnification: (a and b) × 200; (c and d) × 400.