Regeneration of sensory cells after laser ablation in the lateral line system: hair cell lineage and macrophage behavior revealed by time-lapse video microscopy.

The regeneration of sensory hair cells in lateral line neuromasts of axolotls was investigated via nearly continuous time-lapse microscopic observation after all preexisting hair cells were killed by a laser microbeam. The laser treatments left neuromasts with one resident cell type, which was supporting cells. Over the course of 1 week, replacement hair cells arose either directly via differentiation of cells present in the epithelium from the beginning of the time-lapse period or via the development of cells produced after one or two divisions of supporting cells. All of the cell divisions that produced hair cells were asymmetrical. During the first hour after the treatment, macrophages and smaller leukocytes were attracted to the laser-treated neuromasts. The smaller leukocytes returned to control levels 48-60 hr after the treatment, whereas macrophages remained active there throughout the period of hair cell replacement. Macrophage incidence peaked 36-48 hr after the laser treatment. Macrophages phagocytosed damaged hair cells and supporting cells, as well as new cells and preexisting cells without recognizable damage. The results provide direct evidence of hair cells arising as progeny produced from the divisions of supporting cells, evidence of hair cells and supporting cells arising from the same cell division, evidence relating to the timing of hair cell differentiation, and indirect evidence pertaining to proposals that hair cells sometimes arise via conversion of cells without an intervening division. The results also suggest that macrophages may influence early stages in the process of hair cell regeneration.