Chapter 7 - Small and Large Intestine

Abstract

The small and large intestines provide an important barrier against ingested, inhaled, and swallowed toxic agents. Conversely, the intestine is of major importance because it represents the primary site of absorption of toxic or carcinogenic substances. It possesses an enormous surface area of specialized absorptive epithelium, evidenced by its length and the presence of villi. In addition to the intestine acting as an absorptive surface, it also plays an important role in the metabolism of compounds. Biotransformation and activation or deactivation can occur if compounds are ingested or if they reach the intestine through enterohepatic circulation. The gastrointestinal microflora not only possesses a metabolic capacity, but also can influence the turnover rate of mucosal cells and subsequent exfoliation and release of enzymes into the lumen. Evaluation of the intestinal tract for toxicity should be conducted using macroscopic, microscopic, and ultrastructural methods. In carcinogenicity studies, it is imperative that the intestinal tract be opened and the mucosal surface of the entire length be examined for neoplasms or other gross lesions. In the absence of gross lesions at necropsy, standard transverse sections of small and large intestine are prepared for microscopic examination.