ABSTRACT
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Components of the gastrointestinal (GI) tract serve as the rst interface of a host organism with xenobiotics by oral dosing. The tract may be affected by this initial exposure but may also be impacted by systemic exposure via the vasculature or due to enterohepatic circulation. Additionally, the GI tract is important in maintaining the general health of the organism if for no other reason than its role in providing nutrients for the host; furthermore, the intestine has signicant metabolic activities on its own (similar to liver), and its luminal contents contain modifying factors for xenobiotics (such as bacteria and bile salts) as well as dietary components, including ber. Its gross structure varies by species, primarily inuenced by diet and its biochemical features such as enzyme activity; cell products including mucus contents may be altered by environmental factors including bacteria and diet along with genetic inuences. Perhaps due to the vulnerability to many of these external inuences, much of the GI tract has a rapid turnover rate. The high metabolic activity makes many of its cells (especially the surface epithelium) and the GI tract as a whole sensitive to hypoxia, which is reected in functional/morphological changes in the living organism and presents a challenge to pathologists because of rapid autolysis. This chapter provides information on the integrated assessment of this complex organ system in the most commonly used toxicologic species, from rodents (mice and rats) to large animals (dogs and nonhuman primates). Detailed evaluation by the toxicologic pathologist requires familiarity with the anatomic and physiologic differences along with molecular events and conventional morphological characteristics of various tissues, including their pathological responses.
