Morphologic Considerations in the Interpretation of Gastrointestinal Disorders

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Abstract
Misinterpretations of gastrointestinal alterations can be reduced or eliminated by avoiding sampling/labeling errors, reducing artifact, and gaining familiarity with normal variations for the species under study. The errant thrust of the gastroscopic biopsy forceps can yield mucosa resembling atrophic gastritis; ileal villi taken for jejunal erroneously lead to consideration of sprue-like disorder. Reduction of the postmortem delay prior to fixation is critical for avoiding artifact. Villous epithelial cells separate from basement membrane within 5 to 10 minutes. Flushing saline, water, or fixative through unopened intestine causes intolerable damage to epithelium and villi. The thin-walled intestines of small laboratory species may be fixed unopened, but without dissection gross lesions are overlooked. Osmolarity of the fixative is important for preserving normal shapes of villi. Smooth muscle of the muscularis and villi of the dog contracts vigorously after euthanasia and immediate fixation. Optimum preservation may be had by applying iso-osmolar neutral buffered fixatives to intestinal segments which have been opened, stretched slightly, and pinned to cardboard or styrofoam. The villous height: crypt depth ratios vary among species and among intestinal segments. Shapes vary as well, ileal villi of many species appearing short and Christmas-tree- or thumb-shaped. Dilated lacteals occur occasionally in normal villi. The goblet cell population increases aborally, while the lymphocytic-plasmacytic population of the core increases with age. Eosinophils are a normal component in several species; globule leukocytes are common in others. Mast cells of the villi will be overlooked unless tissues are fixed in Carnoy's solution and stained with Astra or Alcian blue. Solitary lymphoid follicles, jejunal Peyer's patches, ileal Peyer's patches, and colonic lymphoid areas must each be evaluated with an understanding of the dome epithelium, B-cell germinal centers, T-cell zones, and surrounding lymphatic sinusoids.