Ectopic stem cell niches sustain rainbow trout (Oncorhynchus mykiss) intestine absorptive capacity when challenged with a plant protein-rich diet

To develop more sustainable feed formulations, it is important to assess in detail their effect on gut function and health. We previously described the specific organization of the epithelial and stromal components of the intestinal stem cell niche (ISCN), in rainbow trout (RT) under actual farming conditions. In the present work, we used our previous observation, for performing a comparative analysis between a control diet (CF) and an experimental vegetable-based diet (CV) under a new perspective. We correlated diet-induced changes of the morphology and the absorptive capability of the RT mucosa with modifications of the ISCN. Histological analysis confirmed that CV diet caused a mucosa remodeling, characterized by the generation of accessory branches sprouting from the middle of the proximal intestine folds, determining a significant increase of the luminal surface. The newly-formed structures showed positivity for PepT1, Sglt-1, and Fabp2 indicating their active role in small molecule absorption. However, the cells lining the base of the new branches expressed both epithelial (sox9) and stromal (pdgfrα and foxl1) stem cell markers, rather than the expected markers of fully differentiated cells. Our results suggest that a nutritional challenge results in the formation of an ectopic ISNC at the middle of the intestinal folds that sustains the formation of functional collateral branches, presumably to compensate for the reduced intestinal absorption. Overall, these data highlight, for the first time, the plasticity of the ISCN and its possible role in compensating intestinal functions in response to challenging conditions.