Leukocytes migrate specifically to damaged lateral line neuromasts in zebrafish larvae. (a-j) 56-hours postfertilization (56-hpf) BACmpx::GFP or lysC::DsRED2 transgenic zebrafish larvae exhibit green or red fluorescent leukocytes, respectively. (a and b) Untreated fish show the normal distribution of labeled cells, mostly localized in the ventral trunk and tail. (c and d) In copper-treated siblings, leukocytes become localized preferentially to a few clusters along the horizontal midline of the trunk and tail. (e-j) A detailed view of this region in copper-treated animals shows that while many cells disperse throughout the body, other cells congregate in discrete clusters (arrows); no overt tissue damage to the larvae is observed in bright-field images. (k-r) A mating cross of cldnB::GFP and lysC::DsRED2 transgenic fish labels neuromasts in green and leukocytes in red. Posterior trunk neuromasts were imaged immediately after adding copper (k-n) or 20 minutes after copper treatment (o-r) using bright-field red or green fluorescence illumination. Few, if any, leukocytes are seen near neuromasts at the beginning of treatment. (m and n) Here a case where a single leukocyte is present is shown. (q and r) In contrast, copper-treated fish have numerous red fluorescent leukocytes interspersed within the neuromast cells. Note the extent of damage induced by copper in the neuromast cells (compare Figures 1l and 1p).