Exposure to high ambient temperatures may result in structural changes in the small intestine of rodents, poultry, and livestock [4,19,20,23,24,25,26,27,28,29]. The intestinal epithelium is composed of a single layer of cells that lines the inner surface of the small and large intestine, which, in addition to providing a solid protective barrier, functions as a very precise absorptive machinery. In mammals and birds, this cell line folds along its path through the GIT to increase its contact surface and thus its absorptive power, generating villi (raised portions) and invaginations between villi, which are termed crypts. In vivo models for studying gut physiology have shown hyperplastic crypts combined with reduced villus area, suggesting rapid tissue adaptation to nutrient shortages [30,31,32]. Heat stress may affect the intestinal structure by shortening villus height and increasing crypt depth, with a consequent decrease in the villus:crypt ratio, as shown in poultry [19,20,24,25,33], rodents [21,27], and pigs [22,23] (Table 1). The aforementioned effects have been found in different heat-stressed animal models (i.e., ambient temperatures from 33 to 39 °C for 1.5 to 24 h/day, and duration of insult from 1 to 30 days). Furthermore, heat stress has led to epithelial desquamation at the tips of villi and exposure of the lamina propria in the duodenum and jejunum of pigs (i.e., ambient temperature of 40 °C for 2–5 h) [28,29] and in the jejunum and ileum of rats (i.e., ambient temperature of 40 °C for 2 h/day for 10 days [26] or 3 days [34]). Although the direct effect of heat may result in epithelial loss [2], the shift in blood flow away from the gut with a concomitant shortage in nutrient supply to the intestine may contribute to the alterations in intestinal architecture observed in heat-stressed animals.

Changes in the epithelial ultrastructure of the jejunum have been found in animals exposed to heat stress. Electron microscopy analysis revealed that a large amount of inflamed fibrous substances flow out of the hyperthermic rat jejunal epithelium [34]. In pigs and rats, heat stress (40 °C for 2–5 h/day for 10 days) affects epithelial cells of the jejunum, shortens microvillus height and increases the number of mitochondria with shortened internal cristae and secondary lysosomes compared with the jejunum in thermoneutral (TN) animals [26,28,29]. Vacuolization in the epithelium in the jejunum of rats exposed to heat stress has also been reported, possibly in association with the progressive loss of epithelial cells [26]. Although these changes in cellular structure and ultrastructure have been observed in all portions of the SI, it seems that the jejunum has greater susceptibility than other segments of the GIT [28,29,34].

In addition to loss of the epithelium of villi, the metabolic alterations generated by heat stress in the intestinal stem cells at the bottom of crypts may delay epithelial cell turnover and replenishment. Intestinal stem cells exhibit a high regenerative power that ensures the turnover of most mature epithelial cells in less than five days [35]; however, exposure to high temperatures can alter the proliferation and apoptosis of intestinal stem cells. As recently demonstrated by Zhou et al. [36] using in vitro models, continuous heat exposure at 41 °C for 72 h of undifferentiated porcine jejunal epithelial cells inhibits cell proliferation and increases apoptosis via inhibition of the Wnt/β-catenin pathway, the main signal that activates the proliferation of intestinal epithelial cells in the depth of intestinal crypts [37].

Stressful events such as exposure to heat can alter the permeability of the intestine to luminal contents (e.g., nutrients and markers) [2,17,38,39]. As alterations in intestinal permeability may reveal changes in absorptive mechanisms (e.g., paracellular pore and leak pathways), this approach is used to assess intestinal function. Gut permeability can be studied by measuring the passage of markers such as fluorescein isothiocyanate-dextran (FITC-D), creatinine, cobalt-EDTA, lactulose and mannitol in vivo and by measuring transepithelial electrical resistance (TER) and permeability to horseradish peroxidase ex vivo and in vitro. Studies have shown that FITC-D increases while TER decreases in the jejunum and/or ileum of pigs and poultry exposed to heat stress [16,19,25,38,40,41,42], indicating an increase in mucosal permeability. In pigs subjected to heat stress (31 ± 1 °C, 12 h/day for 7 days), it was determined that ileum and colon permeability increased, as indicated by a larger lactulose: mannitol ratio [43]. Furthermore, increased intestinal permeability was observed in mice [2,44] exposed to heat stress. In vitro studies have demonstrated increases in permeability or decreases in TER in cultures of epithelial cells, such as intestinal epithelial cell-6 (IEC-6) [44], porcine jejunal cell line (IEC-J2) [43], and human colon-derived crypt-like cells T84 [45] exposed to heat treatments.

Tight junctions (TJs) and adherent junction proteins play a key role in controlling the permeability of the intestinal epithelium [2,46] and in controlling the passage of nutrients via the paracellular space between adjacent cells [47]. Expression of TJ proteins (i.e., occludin and zonula occludens-1 (ZO-1) in the jejunum [19] of chickens and ZO-1 in the jejunum of dairy cows [17]) was reduced during heat stress, and heat stress reduced expression of occludin and claudin-3 in the ileum of pigs [16]. Such an increase in occludin expression was also reported in a study with Caco-2 cells exposed to heat [48]. Gene expression analyses showed higher mRNA expression of claudin-5 and ZO-1 in the jejunum and claudin-1 and -5 and ZO-1 in the ileum in heat-stressed broilers [49]. It was reported that in pigs, heat stress increases the mRNA abundance of occludin, ZO-1 and claudin genes (i.e., pig jejunum after exposure to constant 35 °C for 7 days [23]); in dairy cows, heat stress increases the mRNA abundance of the ZO-1 and claudin-3 genes (i.e., cow jejunum after constant 28 °C for 4 days [17]). However, mRNA levels of occludin, ZO-1, and claudin-1 genes decrease in the jejunum [19,25] of heat-stressed broilers. These contradictory results can be attributed in part to differences between species and studies concerning the duration and severity of heat exposure. It is possible that permeability changes associated with heat stress are mediated by altering expression of TJs in the intestinal epithelium. This relationship has been described in both in vivo and in vitro models of enteritis, aiming to better understand the role of cellular mechanisms to prevent and treat the clinical conditions of humans. Heat stress may lead to changes in TJ expression in the epithelium, but the role of these changes in intestinal permeability remains elusive.