Mills Ch22 Stomach

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SECTION VII : AlimentaryTract

Metaplasia There are two major types of metaplasia that are seen in the stomach: IM and pyloric (pseudopyloric) metaplasia. Both are thought to be the result of chronic gastritis, and conse- quently, both are more frequently encountered in elderly individuals; neither type is considered symptomatic. In pyloric metaplasia, there is a replacement of the specialized acid- and enzyme-secreting cells of the oxyntic glands by mucus-secreting glands of the type present in normal pyloric mucosa. This change occurs in the zone of oxyntic mucosa adjacent to the histologic fundopyloric junction, and what were typical oxyntic glands now come to resemble typical pyloric glands. Therefore, in persons with extensive pyloric metaplasia, the oxyntic gland area of the stomach contracts, the pyloric gland area expands, and the junctional zone is moved proximally toward the cardia (30). Unless the site of biopsy is known with accuracy, pyloric metaplasia cannot be diagnosed on routine H&E sections. However, although the fundic glands lose zymogenic and parietal cells, they still retain pepsinogen I activity. This can be demonstrated by immunohistochemical methods (34). In IM, there is a change in the cells of the surface and pit epithelia so that morphologically and histochemically they come to resemble the cells of either the small or large bowel; IM may be complete (type I) or incomplete (type II) (33,34). In complete small bowel IM, the gastric mucosa changes to resemble normal small bowel epithelium, char- acterized by fully developed goblet cells and enterocytes with a brush border (Fig. 22.21). In advanced cases, the contour of the mucosa changes with the development of villi and crypts. Paneth cells may be present in the base of the crypts. In incomplete metaplasia, recognizable absorptive cells are not seen. The epithelium consists of a mixture of intestinal-type goblet cells and columnar mucus-secreting cells, morphologically resembling those of the normal gastric epithelium.

FIGURE 22.22  Complete intestinal metaplasia (PAS/Alcian blue).

Histochemical changes detected in the carbohydrate composition of mucus in the various types of IM are inter- esting and complex (17,35). In the normal stomach, mucus secreted by the columnar cells is neutral in type, recognized histochemically as PAS positive and Alcian blue negative. In complete IM, the enterocyte cytoplasm, apart from the brush border, is mucin negative, but the goblet cells secrete either sialomucin (an acid mucin that is PAS positive, Alcian blue positive at pH 2.5, but Alcian blue negative at pH 0.5) or sulfomucin (a strongly acidic mucin that is weakly PAS positive and Alcian blue positive at pH 2.5 and at pH 0.5) (Fig. 22.22). In incomplete small bowel metaplasia, sialo- mucin is present in the columnar cells, and in incomplete large bowel metaplasia (also called type III metaplasia) (34), the columnar cells contain sulfomucin (Fig. 22.23). Sulfomucin may be recognized separately from sialomucin because it stains positively with high-iron diamine (36). The details of these methods are well described in standard textbooks of histochemistry (37).

FIGURE 22.23  Incomplete large bowel metaplasia. The pit contains columnar cells with cytoplasmic sulfomucin (high-iron diamine and Alcian blue).

FIGURE 22.21  Complete intestinal metaplasia (IM).