Figure 6. Macrophage response to mechanical overstimulation of lateral line hair cells.

Experiments used Tg(mpeg1:yfp) fish that express YFP under regulation of the macrophage-specific mpeg1 promoter. All images and data were collected from the two distal-most neuromasts of the posterior lateral line (Figure 2A; term). (A) Macrophages (green) responded to mechanical injury by entering neuromasts, contacting hair cells and internalizing Otoferlin-immunolabeled debris (arrows, magenta). Images show examples of macrophage behavior at different time points after noise trauma. (B) Quantification of hair-cell number in the terminal neuromasts. Hair-cell number was significantly reduced at 0–2 hr after noise exposure (Mixed-effects analysis: **p < 0.003). (C) Quantification of macrophages within a 25 µm radius of the neuromasts at 0–8 hr after noise injury. Most neuromasts possessed 1–2 nearby macrophages and this number was not changed by noise exposure. (D) Quantification of direct contacts between macrophages and hair cells. The number of macrophage-hair cell contacts was counted at each survival time after noise exposure and normalized to the total number of sampled neuromasts. Increased levels of contact were observed at 2 and 4 hr after noise (*p = 0.0243). (E) Quantification of phagocytosis as a function of post-noise survival time. The numbers of macrophages that had internalized otoferlin-labeled material were counted at each time point and normalized to the total number of sampled neuromasts. The percentage of macrophages that contained such debris was significantly increased at 0–2 hr after strong water wave stimulus (*p = 0.0465; **p = 0.0013). Data were obtained from 26 to 50 neuromasts/time point. Error Bars = SD.