Which statement is correct about ASBT labeling in control dogs?

• A. Ileum labeling absent; colon and cecum labeling continuous in all enterocytes.
• B. Ileum labeling restricted to crypts; colon and cecum labeling absent.
• C. Ileum labeling multifocal; colon and cecum labeling continuous and homogeneous.
• D. Ileum labeling continuous and homogeneous in villi enterocytes and absent in crypts; colon and cecum labeling multifocal and restricted to superficial enterocytes.

Answer: (D)

ASBT expression in healthy dogs follows a clear pattern along the intestinal lining and between segments. In the ileum, ASBT sits on the apical membranes of mature enterocytes along the entire length of the villi, so labeling is continuous and homogeneous in villus enterocytes and absent in crypt enterocytes. The colon and cecum have much lower ASBT expression; labeling there is multifocal and restricted to superficial enterocytes, not extending into the crypts. This distribution fits the biology that most bile acid reabsorption occurs in the ileum via apical ASBT on villus cells, while the colon/cecum reabsorb far less, in a patchy, superficial manner. Hence, continuous ileal labeling in villus enterocytes with no crypt labeling, and multifocal, superficial labeling in colon/cecum, is the correct pattern for control dogs.