3.4.1. In Situ Hybridization

In the rostral intestinal bulb of young fish, NUCB2B mRNA was detected in the lining epithelium. Signal was restricted to the columnar shaped enterocytes of the apical regions of the ridges (Figure 7C,D). NUC2B mRNA distribution did not show notable differences between young and old subjects (Figure 7E,F).

#### 3.4.2. Immunohistochemistry

Cross sections of the rostral intestinal bulb of young fish showed Nesf-1 positive rounded or flask-shaped cells in submucosa, mainly scattered deep within the folds of the ridges (Figure 7G,H). In old fish, Nesf-1 ir-cells were more abundant in the submucosa and, in addition, some of the cells were dispersed along the apical regions of the ridges (Figure 7I,J). The number of positive cells did not reveal differences between the young and old animals (Figure S1).
