Effects of fixation on electrophysiology and structure of human jejunal villi.

The villi of human jejunum vary in size and shape during different functional conditions. In the base the lamina propria is isotonic with blood, in the tip hyperosmotic. Here we study electrophysiological and morphological effects of incubation in hypotonic, isotonic, or hypertonic solutions, and to test various isotonic fixatives for microscopy. Samples of jejunal mucosae, obtained during surgery in obese patients, were studied in Ussing chambers where electrical parameters were registered during incubation in Krebs solution at various osmolarities, and during fixation in formaldehyde, glutaraldehyde, or osmium tetroxide (OsO4 ). The same fixatives were used for other jejunal specimens that were fixed directly for light microscopy. Morphometry was carried out to determine size and height of villi, proportion of lamina propria, and surface enlargement due to villi. Ussing chamber incubation in fluids with low osmolarity resulted in increased electrical resistance and epithelial swelling. Opposite results were obtained at high osmolality. Fixation was faster in formaldehyde than in glutaraldehyde or OsO4 . In biopsies processed directly for light microscopy the proportions of lamina propria of the mucosa, and of lamina propria of villi, were significantly larger in biopsies fixed in formaldehyde than after fixation in glutaraldehyde or OsO4 . The villus tips sometimes ended with a bleb with prominent spaces between the epithelial cells. In summary, jejunal villi swell in vitro when exposed to hypotonic solutions, and shrink in hypertonic solutions. Much of the morphological changes occurring during fixation can be related to the physiological hyperosmolar milieu in villus tips.