Neuroepithelial cells (NECs) in gills, skin, and respiratory gas bladder in fish are neurosecretory and contain serotonin (5-HT). (A) NECs of gill filaments (F) and lamellae (L) of adult zebrafish were labeled with antibodies against 5-HT (green), and innervating nerve fibers were labeled with a neuron-specific marker (zn-12, red). Scale bar = 50 µm. Confocal microscopy and immunolabeling procedures were similar to those described in Ref. 22. (B) NECs and associated nerve fibers of the yolk sac epithelium of a 7-dpf zebrafish larva were labeled in the same manner as in A. Note the long processes (arrow) that extend to make contact with the external environment. Scale bar = 20 µm. (C) Transmission electron micrograph of two NECs from Lepisosteus oculatus respiratory bladder showing heterochromatin nucleus (asterisks), small mitochondria (arrowheads), numerous profiles of smooth and rough endoplasmic reticulum, and specific, membrane-bound, irregularly shaped vesicles with dense content (arrows). Note their close relationship with nerve bundle (NB)-containing myelinated axons. Scale bar = 1 µm. Modified with permission from Ref. 48. (D) Immunocytochemical double staining for calbindin D28k (red) and 5-HT (green) of a NEC in the mucociliated epithelium of the respiratory gas bladder of L. oculatus. The confocal image shows a combination of the two color channels and the orientation of the cell making contact with the external surface by a slender process (arrow). GC, goblet cell; C, ciliated cell. Scale bar = 10 µm.

Summarized data of hypoxia/anoxia tolerance in developing zebrafish (Danio rerio) at different oxygen partial pressures. Pie charts show the percentage of survivors (black) to nonsurvivors (gray) at different ages and incubation protocols. Brackets indicate multiple data at the same time point. Horizontal bars depict survival time windows of differently affected cardiovascular convection capacities and changes of main gill function during early development. (Hypocytemic or hypochromic anemia does not affect survival during the first 10 dpf [T. Schwerte, unpublished observations]). Photograph inset shows a paralyzed (immobile) 3-day-old larva after an anoxic episode with digital motion-contrasted vessels showing functional circulation. Orange encodes a higher blood velocity (heart and dorsal artery) compared to medium velocity (green). # indicates data on mutant: breakdance (bre). * indicates data on mutant: silent heart (sih). Data were taken from Refs. 11, 78, 79, 80, 83, 84, and 86.