Modulation of intestinal growth and differentiation by photoperiod and dietary treatment during smoltification in Atlantic salmon (Salmo salar, L.)

Atlantic salmon undergo smoltification, a process that prepares the fish to enter and thrive in seawater (SW). Several physiological changes occur during smolting, especially in osmoregulatory tissues, the gill, the kidney, and the intestine. Here we characterized the effects on intestinal morphogenesis of two different, commonly used smoltification regimes during the end of the freshwater phase, photoperiod and/or the addition of salt and amino acid supplements in the diet. We focused on intestinal morphological differentiation, i.e., external perimeter, absorptive perimeter, tissue thickness, and villi density. In addition, we quantified cell proliferation (PCNA positive) and Na+, K+-ATPase (NKA) and Na+, K+,2Cl− (NKCCs) co-transporters expression and enterocyte apicobasal distribution by immunohistochemistry. These analyses show that the anterior and posterior intestines have different developmental dynamics during smoltification. In both intestinal regions, photoperiod and dietary treatment increased the absorptive perimeter. In addition, diet and photoperiod treatments differentially stimulated NKA protein expression in the anterior intestine. NKCC apical-basolateral expression in the enterocytes increased after SW entry in the anterior and posterior intestines. In conclusion, our results show that, as smoltification progresses, the anterior intestine responds more readily to experimental conditions than the posterior intestine. In our study, photoperiod and dietary treatment seem to enhance the development of the capacity to tolerate SW.