OHCs lacking mucolipins 3 and 1 possess pathologically enlarged lysosomes. A, Immunoreactivity of LAMP1 (a lysosomal membrane protein) in adult (∼P120) OHCs reveals apical accumulation of lysosomes in DKO OHCs but not in those from WT, ML1KO, and ML3KO control cochleae. B, Super-resolution structured illumination microscopy shows that DKO OHCs, but not the controls, possess enlarged lysosomes as indicated by LAMP1 immunoreactivity. B, Images were OHCs from row 2. C, Quantification of total whole-cell LAMP1 fluorescence from A reveals a significant increase in LAMP1 in DKO OHCs but not in control cochleae. D, LAMP1 whole-cell intensity values used in C are replotted separately based on cochlear position (apical, apical/middle, and middle). We observed no difference in OHC LAMP1 intensity between cochlear turns within each genotype. E, LAMP1 intensity values used in C are replotted, binning the apical-basal axis of OHCs in three segments based on the percentage distance from the nucleus. F, By measuring LAMP1 vesicle diameter from images taken from confocal and structured illumination microscopy, we found that lysosomal diameter of DKO OHCs is ∼2.18-fold larger than that from control OHCs. G, A frequency distribution plot of LAMP1 vesicle diameter in all genotypes. Scale bars: A, 5 μm; B, 2 μm. *p < 0.05; **p < 0.01; ***p < 0.001; ****p ≤ 0.0001.