Histological analysis of small bowel biopsies shows a variable degree of villus atrophy without any inflammatory infiltrate. Highly characteristic for this disorder is the accumulation of periodic acid Schiff (PAS)-positive secretory granules within the apical cytoplasm of enterocytes.18-20 On
the ultrastructural level, rare or absent microvilli on intestinal epithelial cells along with inclusions of microvilli in the cytoplasm of enterocytes are seen, which define this entity.21
Diagnosis may be easily performed from light microscopic examination of a duodenal or jejunal biopsy specimen. On hematoxylin-eosin staining, the mucosa appears flattened with hypoplastic villus atrophy. PAS staining reveals an abnormal brush-border pattern with positive-staining material within the apical cytoplasm of enterocytes (Figure 1.1). A valuable new tool for the light microscopic diagnosis of MVID was recently proposed.22,23 CD-10 is a membrane-associated neutral peptidase, shown to have a linear brush-border staining pattern in normal small intestine. In contrast to this surface staining in different controls (normal intestine, celiac disease, autoimmune enteropathy, allergy), all MVID cases revealed prominent intracytoplasmic CD-10 immunoreactivity in surface enterocytes22,23 (Figure 1.2). Similar results were obtained with PAS, polyclonal carcinoembryonic antigen and alkaline phosphatase, three stains known to show cytoplasmic staining of surface enterocytes in MVID.23 On electron microscopy, surface epithelial cells show absent or grossly abnormal microvilli, as well as numerous vesicular bodies of various sizes, and the characteristic microvillus inclusions (Figure 1.3). Crypt cells are morphologically almost normal, but do not contain increased
Figure 1.1 Microvillus inclusion disease: periodic acid-Schiff (PAS) staining. (a) Normal mucosa, normal PAS, brush-border staining; (b) abnormal accumulation of PAS, positive material in the apical cytoplasm of epithelial cells in microvillus inclusion disease.
numbers of apical vesicles and vesicular bodies. Microvillus inclusions, as well as increased secretory granules, are also present in the large bowel, more easily accessible for biopsy, especially in early infancy (Figure 1.4).
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