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An ultrastructural study has suggested that microvillus inclusion bodies (MIBs) seen in one form of intractable diarrhoea affecting infants arise from increased autophagocytosis of brush borders of enterocytes, and not from disrupted intracellular biosynthetic processes.
No differences were apparent in antibody labelled actin or villin in patients or controls. Biosynthetic labelling showed normal functioning of intracellular processing and transport of the brush border enzyme sucrase-isomaltase (SI). Finally, MIBs took up extracellular cationised ferritin and ovalbumin. Both were endocytosed from the apical surface of enterocytes and then accumulated inside MIBs. The MIBs stained positive for SI protein but did not contain lysosome associated membrane protein.
Frozen thin sections of ileum from four children with microvillus inclusion disease (MID) were double labelled with primary antibody to actin and villin components of the cytoskeleton and immunogold. Sections of biopsy specimens from two patients were incubated with cationised ferritin or ovalbumin before processing to examine endocytosis. Biopsy material from one patient was incubated in organ culture with radiolabelled methionine, then treated to disrupt the membranes. After treatment with antibody against SI protein the precipitate was analysed for SI precursors and mature SI protein.
The events leading to villus atrophy of enterocytes, a prominent feature of MID, are not known for certain but seem to involve MIBs. Accumulation of electron dense secretory granules at the apex of crypt epithelial cells has led to a suggestion that MIBs may result from some defect in intracellular biosynthesis. Components of the cytoskeleton were said to be involved too.