Mitogenic action of hypoxia upon cutaneous neuroepithelial cells in developing zebrafish

In zebrafish, cutaneous neuroepithelial cells (NECs) contain serotonin (5-HT) and are believed to initiate physiological and behavioral responses to hypoxia during embryonic and early larval development, when mature gills and O₂ chemoreceptors are not yet present. The number of skin NECs rapidly declines as embryos develop into larvae, but acclimation to hypoxia leads to retention of a greater number of these cells. We hypothesized that reduction of the partial pressure of oxygen (P_O2 ) in water would stimulate mitosis in cutaneous NECs in zebrafish. Zebrafish were exposed to 5-bromo-2'-deoxyuridine (BrdU) and immunolabeled with antibodies against serotonin and BrdU to identify mitotic skin cells, including NECs. Cells were imaged and quantified using confocal microscopy. From embryonic to larval stages, we observed an overall increase in the number of BrdU-positive cells in the skin, but a decrease in BrdU-positive serotonergic NECs. Exposure of larvae to hypoxia (P_O2  = 30 mmHg) in vivo for 24 h produced a 1.7-fold increase in the number of NECs labeled with BrdU. We conclude that under normal environmental P_O2 the population of cutaneous NECs declines due to a decrease in mitotic activity. During environmental hypoxia, the number of NECs undergoing cell division in the skin is increased, and this promotes retention of NECs under these conditions. These data demonstrate the direct action of hypoxia upon the cell cycle of cutaneous NECs in developing zebrafish, and support the notion that cutaneous NECs are embryonic O₂ chemoreceptors. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 77: 789-801, 2017.