Further evidence of species variation in mechanisms of epithelial cell loss in mammalian small intestine: ultrastructural studies on the reindeer ( Rangifer tarandus ) and seal ( Phoca groenlandica )

Abstract

Ultrastructural studies were conducted on mechanisms of epithelial cell loss in the small intestine of seal and reindeer. Mechanisms maintaining epithelial integrity were distinguished from those that did not and the non-epithelial cell types involved were identified. Three types of cell extrusion were noted. In two, tight junctional integrity was preserved and anucleate apical cell fragments (rather than complete cells) were lost into the lumen. In reindeer (type 1), this involved creating large intercellular spaces extending from the preserved apical cap to the lamina propria and containing enterocyte debris probably phagocytosed by subepithelial macrophages. A variant of this process (type 2) involved the gradual shrinkage of individual cells, which became more electron-dense, and the in situ degeneration of their nucleated subapical portions. Degenerated cell fragments and membrane whorls were confined to narrow intercellular spaces between approximating adjacent healthy enterocytes. The mechanism of removal of these fragments was unclear. In both cases, the proximity of intraepithelial lymphocytes suggested that they were involved in cell targetting and killing. Evidence of apoptotic nuclei was not found but nucleated cell fragments could have been washed out of the lumen during tissue preparation. Type 2 cell loss was seen in both species, as was another mechanism (type 3) reminiscent of necrosis. In contrast to other mechanisms, this was accompanied by breaks in epithelial continuity following gradual loss of cell electron density and total or subtotal degradation of organelles and membranes. In seal, this terminated in the loss of an abnormal cell apex and exposure of the contents of the cell remnant to the lumen. In reindeer, all the cell remnants may have been extruded before total membrane degeneration but, in both species, the otherwise tight epithelial barrier was clearly breached. Again, intraepithelial lymphocytes were associated with sites of necrosis. These findings provide evidence for further species differences in mechanisms of epithelial cell extrusion and suggest that necrotic cell loss may be more common than previously admitted.

Early Epithelial Invasion bySalmonella entericaSerovar Typhimurium DT104 in the Swine Ileum

“…26 Salmonella serovar Typhimurium invasion occurring at sites of enterocyte loss may be especially relevant because swine and other species have been suggested to have a type-3 cell loss mechanism. 14,16 This type of cell loss tends to occur in isolated individual enterocytes, allowing for breeches of integrity in the tight junction. These cells exhibit morphologic changes similar to oncotic necrosis and are often associated with intraepithelial lymphocytes and possibly neutrophils.…”

“…These cells exhibit morphologic changes similar to oncotic necrosis and are often associated with intraepithelial lymphocytes and possibly neutrophils. 12,16,18,27 In the mouse, individual cell loss can occur over the entire villus and may exhibit a circadian variation. 20 In this experiment the extrusion frequency increases over time similarly in both treatment and control loops.…”

Effects of natural winter pasture and commercial pellet on the ultrastructure of small intestinal epithelium in reindeer

“…Three samples of three small intestinal regions (duodenum, jejunum and ileum) from three animals grazing natural winter pastures and from five reindeer fed the commercially available pellet RF-80 were cut into small pieces suitable for transmission electron microscopy (TEM) and fixed in McDowell's fixative (McDowell and Trump 1976). After fixation, samples were further prepared as described elsewhere (Myklebust and Mayhew 1998). Ultrathin sections were contrasted with uranyl acetate and lead citrate and examined with a Jeol 1010 transmission electron microscope (Tokyo, Japan) at an accelerating voltage of 80 kV.…”

“…The small intestine of Norwegian reindeer is about 22 m long and its effective absorptive surface area is amplified by both villi and microvilli. As in other mammals, the villi are covered by a continuously renewing epithelium in which cell turnover is relatively rapid taking only a few days (Myklebust and Mayhew 1998). The small intestinal pH increases from 5.9 in the duodenum to 6.1 in the jejunum and 7.5 in the ileum (Sørmo and Mathiesen 1993;Sørmo et al 1994).…”

“…However, little information is available on the ultrastructure of the small intestine of reindeer (Myklebust and Mayhew 1998). The small intestine of Norwegian reindeer is about 22 m long and its effective absorptive surface area is amplified by both villi and microvilli.…”