Zebrafish lateral-line neuromasts. (A) Schematic depicts a larval zebrafish. Pink patches indicate the location of hair cells in the inner ear required for hearing and balance, as well as hair cells in the lateral-line system. Green patches represent the location of the anterior and posterior lateral-line ganglia. The cell bodies of neurons in these ganglia project to and innervate hair cells in the lateral line. (B) A side view of the anatomy of a single lateral-line neuromast. Hair cells (pink) are surrounded by supporting cells (internal, blue and peripheral, orange) and innervated by both afferent (green) and efferent neurons. Mechanosensory hair bundles (purple) at the apex of hair cells project out into the water to detect local water flow. Mitochondria (yellow, orange) make up dynamic tubular networks within hair cells. Adapted from Kindt and Sheets (2018).

MitoTimer fluorescence in hair cells. (A, B) Maximum-intensity projections of hair cells from Tg[myo6b:mitoTimer]^w208 fish at 3dpf (A) and 5dpf (B). (C) Mean mitoTimer fluorescence plotted as ratio of red:green at 3, 4, and 5 dpf. Scale bar, 5 μm. Reproduced with permission from Pickett et al. (2018).

Diagram of Ca²⁺ signaling in hair cells exposed to neomycin. Neomycin enters the hair cell through MET channels at the apex. Inset shows flow of Ca²⁺ from the ER to the mitochondria, resulting in increased ROS levels and increased ΔΨ_m, which leads to hair-cell death. Created with BioRender.com.

Representative images of hair cells expressing transgenes used to visualize mitochondrial dynamics. Live hair cells expressing RGECO1 (A), MitoGCaMP3 (B), and Rex-YFP (C). Scale bar, 5 μm. Adapted with permission from Wong et al. (2019).