Villus tips and crypts are regarded as the anatomical sites of physiological absorption and secretion, respectively. Fluid transport is a bidirectional process in the healthy animal, with net absorption in health and net secretion in disease. The proximal small intestine is relatively leaky; in contrast, the terminal ileum and the colon are powerfully absorptive organs. The balance between absorption and secretion is poised at different points throughout the intestinal tract, which reflects differences in both structure and function of gut mucosal epithelia. Overall control of normal physiological ion/fluid movement is a highly complex phenomenon; for a review of the mechanisms underlying the neural and hormonal regulation of mucosal ion transport in the pig intestinal tract see reference 4. ventolin inhaler
Intestinal epithelia are themselves dynamic structures that undergo complete turnover in approximately three to five days. Crypts are the principal sites of cell regeneration, and villus tips are the sites from which senescent cells are shed to be replaced with cells of crypt origin. Cells differentiate into absorptive enterocytes while migrating up the epithelial escalator.
Finally, in addition to the systems outlined above, neonatal brush borders contain disaccharidases (principally lactase [Figures 1 and 4a]), which break down otherwise nonabsorbable disaccharides (eg, lactose) into constituent absorbable monosaccharides. As discussed below (see section on rotavirus), excessive loss of disaccharidase function may lead to an osmotic purge arising from nonabsorbed lactose.
Diarrheal disease can result from interference with almost any one or combination of these systems, as the rest of this article seeks to illustrate.
Figure 4) Lactase expression in the villi of neonatal mice. Frozen sections of neonatal mouse gut stained, fixed and counterstained as described by Collins et al. Lactase appears blue, and the rest of the tissue appears red. a) Ten-day-old normal mouse. b) Neonatal mouse 72 h after infection with mouse rotavirus. Note the residual traces of blue staining indicative of lactase. c) Recovery of lactase expression 120 h after infection with mouse rotavirus