Pathophysiology of rotavirus diarrhea. The enterocytes lining
the small intestine are generally divided into two types:
enterocytes and crypt cells (Fig. 1). Villus enterocytes are mature,
nonproliferating cells covering the villi that are differentiated
to digestive and absorptive functions. The absorptive
enterocytes synthesize a number of disaccharidases, peptidases,
and other enzymes that are expressed on the apical
surface, where they carry out their digestive functions. Absorption
across the enterocyte barrier occurs both by passive diffusion
of solutes along electrochemical or osmotic gradients
and by active transport. While the majority of water transport
is passive along osmotic gradients, transporters such as the
sodium-glucose cotransporter 1 (SGLT1) transport water
along with solute (42). The crypt epithelium lines the crypts
and is the progenitor of the villus enterocytes. Crypt cells lack
the well-defined microvilli and absorptive functions of the enterocyte
and actively secrete Cl ions into the intestinal lumen.
In the normal animal, the combined activity of the enterocytes
and crypt cells results in a constant bidirectional flux of electrolytes
and water across the epithelium. On the villi, the balance
is toward absorption, and in the crypts, the balance favors
secretion (51).