The terminal web of the duodenal enterocyte.

The terminal web-brush border complex of rodent duodenal enterocytes has been studied by electron microscopy to investigate its structure in relation to currently accepted models of motility in this region. The main adherens zone is composed chiefly of a fine feltwork of 5 to 7 nm filaments, some of which originate in zonulae adherentes. In some cells, this is not a complete layer or sheet. Passing into it from its deep aspect are 10 nm tonofilaments, which also form the basal zone. The filament density in the basal zone is less than that of the adherens zone, and many of the tonofilaments are associated with spot desmosomes. The apical zone contains a loose meshwork of 5 to 7 nm filaments with more filaments lying adjacent to plasmalemmae of the zonula occludens. The core of each microvillus contains a bundle of 17 to 48 microfilaments, 5-7 nm in diameter, apparently attached to the apical plasmalemma and with some slender cross filaments between core filaments and the plasmalemma. In the main, these core bundles of microfilaments pass deeply into and often through the adherens zone of the terminal web where they terminate abruptly. Filaments of the terminal web appear to interconnect microfilaments of adjacent core bundles but without positive evidence of 'splaying' of microfilaments of a core bundle within the adherens zone. These findings are discussed in relation to movement of microvilli.