The gastrointestinal tract (GI) can have many abnormalities associated with it. There are various lesions occurring in the GI tract of genetically engineered mice. Some of these lesions are unique to the particular mouse line and some of them resemble human diseases. Thus, it is important to properly interpret these changes. Many of the GI diseases studied in transgenic mice involve the small intestinal tract and large intestinal tract or colon. In all cases, a thorough examination of the GI tract at necropsy is essential. The intestinal tract and cecum are first examined grossly from the serosal surface. One should check for enlargement of Peyer's patches or any masses within the lumen. Peyer's patches can be an early site of lymphoma. The entire length of the small and large intestine should be examined and opened to look for lesions such as tumors, polyps, adenomas, and carcinomas. Tumors should be described, counted, and placed in fixation. Later, they should be embedded and prepared for histopath. It is also important to section portions of the intestinal tract for histopathology.16 The specimen can be taken as a cross section of the bowel or the preferred method of rolling. The intestinal "Swiss" rolls are a continuous segment of bowel. They are prepared by removing the intestines intact and gently inflating with fixative by intraluminal injection and preparing as an intestinal roll (by placing it on an index card and rolling it in a flat spiral around a central toothpick) and fixed by immersion.
Several strains of transgenic or gene-targeted mice develop chronic intestinal inflammation or tumors. These mice make an excellent model for the study of cellular and molecular makeup of preneoplastic stages of intestinal tumorigenesis. Thus, gross pathology and histopathology are critical for assessing preneoplastic and neoplastic lesions. Aberrant crypt foci (ACF) are possible precursors for colon cancer. They serve as excellent biomarkers for preneoplastic lesions. At necropsy, the small and large bowel should be longitudinally cut open and flushed with saline. The samples are fixed in formalin x 24 h then dipped in 0.2% solution of methylene blue in distilled water and rinsed. Using a light microscope at 40x, the samples are examined mucosal side up for ACF. ACF are distinguished by their increased size, prominent epithelial cells, and increased pericryptal space from surrounding normal crypts.41 GEM are also important models for microbial-based diseases such as Helicobacter infection. Microbial infections are often linked to inflammatory diseases and tumorigenesis in the large and small bowel.
Mice are important models for the study of hepatic disease. There have been many models developed for use in detecting pathological and phenotypic expression of disease. They are especially useful for their susceptibility to developing liver cancers. Transgenic technology has had a tremendous impact on the study of human cancers, where aberrant gene expression is often found. As with any study, hepatic assessment starts with data about strain, mating, date of birth, and any clinical signs noted. It is important to keep careful records of individual mouse weights. One should specify if the weight is obtained before or after any type of fasting and before or after exsanguination. It is also important to perform abdominal palpation to assess for any outward signs of hepatomegaly or neoplasia. Clinical evaluation includes sampling blood for clinical pathology assessment. A clinical pathology assessment includes a complete blood count (red blood cell count, pack cell volume, hemoglobin, platelets, white blood cell count, white blood cell differential) and serum chemistries (albumin, alkaline phosphatase, alanine transaminase, urinary bilirubin, blood urea nitrogen, calcium, creatinine, cholesterol, glucose, phosphorus, total protein, Na, K, and Cl). Urine should be obtained for a urinalysis.
A complete necropsy should be performed. Each mouse should be weighed prior to starting the necropsy. The liver is accessed as previously described in the pathology techniques section. Again, it is important to remember that the liver is divided into four lobes and that each must be carefully removed. After complete excision, the entire liver should be weighed, and the relative liver weight can be calculated as a percent (liver weight x 100/body weight).16 The liver should be examined and characterized grossly. The size, color, consistency, and cut surface appearance should be noted and recorded. Any nodular lesions should be described, measured, and recorded for each lobe. When nodules are found on the liver, it is also important to check for local invasion and for metastases elsewhere in the body. The most common sites for metastases include the lung and kidneys. As with any organ, the liver should also be characterized by histopathology. One should be careful to excise slices from each liver lobe and consistently sample from the same section or position of each lobe. This is especially important for DNA synthesis from histologic samples. Other techniques such as special stains or tests for the presence or absence of specific liver enzymes can be used to further identify and characterize lesions.
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