Resorptive Epithelia

Abstract

The ultrastructures of small intestinal enterocytes, which line the villi and are continually renewed, mirror their main tasks in the absorption of digested food. Panels a and b show the apical cell domain of absorptive cells with the prominent brush border made up of numerous densely packed microvilli, which are the sites of final digestion and uptake of nutrients. Microvilli exist in an approximate number of 3,000 per cell, which greatly increases the luminal cell surface. They measure 1–2 μm in length and contain a filamentous core, composed of actin filaments, crosslinked by fimbrin and villin, and attached to the plasma membrane by myosin I and calmodulin. The rootlets of the filament bundles project into the terminal web located beneath the brush border (tw). They are interconnected by intestine-specific spectrin, attach to cytokeratin intermediate filaments of the terminal web, and are part of the apical cytoskeletal apparatus that is responsible for maintaining the upright positions of the microvilli and the overall organization of the brush border (cf. also Fig. 88). Components of the terminal web are associated with the junctional complex. In particular, actin filaments connected with the belt desmosome in the middle part of the junctional complex (panel b-2; cf. also Fig. 98) contribute essentially to the cells-cells expanding motion system. This is responsible for changes of the diameter of the apical cell domains, leading to a tilting of the brush border microvilli, which facilitates contact with the digested nutrients and supports absorption.