Aquatic vertebrates possess diverse types of sensory cells in their skin to detect stimuli in the water. In the adult zebrafish, a common model organism, the presence of such cells in fins has only rarely been studied. Here, we identified scattered serotonin (5-HT)-positive cells in the epidermis of the caudal fin. These cells were distinct from keratinocytes as revealed by their low immunoreactivity for cytokeratin and desmosome markers. Instead, they were detected by Calretinin (Calbindin-2) and Synaptic vesicle glycoprotein 2 (SV2) antibodies, indicating a calcium-regulated neurosecretory activity. Consistently, electron microscopy revealed abundant secretory organelles in desmosomenegative cells in the fin epidermis. Based on the markers, 5-HT, Calretinin and SV2, we referred to these cells as HCS-cells. We found that HCS-cells were spread throughout the entire caudal fin at an average density of 140 cells per mm 2 on each fin surface. These cells were strongly enriched at ray bifurcations in wild type fins, as well as in elongated fins of another longfin mutant fish. To determine whether hydrodynamics play a role in the distribution of HCS-cells, we used an interdisciplinary approach and performed kinematic analysis. Measurements of particle velocity with a fin model revealed differences in fluid velocities between bifurcated rods and adjacent non-bifurcated regions. Therefore the accumulation of HCS-cells near bone bifurcations may be a biological adaptation for sensing of water parameters. The significance of this HCS-cell pattern is reinforced by the fact, that it is reestablished in the regenerated fin after amputation. Regeneration of HCS-cells was not impaired by the chemical inhibition of serotonin synthesis, suggesting that this neurotransmitter is not essential for the restorative process. In conclusion, our study identified a specific population of solitary paraneurons in the zebrafish fin, whose distribution correlates with fluid dynamics.

The human heart is a poorly regenerative organ and cardiac tumors are extremely rare. The zebrafish heart can restore its damaged myocardium through cardiomyocyte proliferation. Whether this endogenous capacity causes a susceptibility to neoplasia remains unknown. Here, we established a strategy to conditionally express the HRASG12V oncogene in zebrafish cardiomyocytes. The induction of this transgene in larvae or adult animals resulted in heart overgrowth with abnormal histology. The malformed ventricle displayed similar characteristics to the regenerative myocardium, such as enhanced cell-cycle entry, incomplete differentiation, reactivation of cardiac embryonic programs, expression of regeneration genes, oxidative metabolism changes, intramyocardial matrix remodeling and leucocyte recruitment. We found that oncogene-mediated cardiac tumorigenesis and cryoinjury-induced regeneration involve TOR signaling, as visualized by phosphorylation of its target ribosomal protein S6. The inhibition of TOR by rapamycin impaired regeneration and rescued from neoplasia. These findings demonstrate the existence of common mechanisms underlying the proliferative plasticity of zebrafish cardiomyocytes during advantageous organ restoration and detrimental tumorigenesis.