Morphological analysis of the spiral ligament (SLi): age-related SLi alterations are observed in OHFD mice. (A) The schema of the SLi. The fibrocytes of SLi are divided into five types. (B) The immunostaining of types II and IV in the SLi using an anti-Na, K-ATPase alpha 1 antibody at 6 months after experiment initiation. The dagger symbol indicates a decrease in Na, K-ATPase alpha 1 expression in the cochlea of OHFD mice after the commencement of the experiment. (C) The immunostaining of type I and V cells in the SLi using the anti-Cx26 antibody at 3 and 6 months after experiment initiation. The double dagger symbol indicates a decrease in Cx26 expression in the cochlea of the OHFD mice after the commencement of the experiment (Bars: 100 μm; YLFD, low-fat diet-fed 3-month-old mice; OLFD, low-fat diet-fed 6-month-old mice; YHFD, high-fat diet-fed 3-month-old mice; OHFD, high-fat diet-fed 6-month-old mice; YHFD-MNAM, high-fat diet plus 1% N1-methylnicotinamide-fed 3-month-old mice; OHFD-MNAM, high-fat diet plus 1% N1-methylnicotinamide-fed 6-month-old mice; OCs, the organ of Corti; SGC, spiral ganglion cells; SLi, spiral ligament; SV, stria vascularis).