Distribution and restoration of serotonin-immunoreactive paraneuronal cells during caudal fin regeneration in zebrafish

Abstract

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.

Regeneration of the dermal skeleton and wound epidermis formation depend on BMP signaling in the caudal fin of platyfish

“…Whether this inverse architecture might affect hydrodynamic properties that regulate ray bifurcation requires further interdisciplinary studies. Indeed, our collaborative projects have previously suggested that fluid dynamics might provide cues for the ray-interray distribution of sensory cells and for the position of ray bifurcations in the regenerating zebrafish fin (Puri et al, 2017;König et al, 2019;Dagenais et al, 2021). In agreement with the hypothesis of the extrinsic input for ray morphogenesis, our experiments with ray excision triggered a distal shift of ray branchpoints.…”

Osteoblasts pattern endothelium and somatosensory axons during zebrafish caudal fin organogenesis

“…For example, a monolayer of bone-depositing osteoblasts covers each hemiray; an artery runs through the middle of each ray, while veins flank the dorsal and ventral margins of each ray (Xu et al, 2014); and previous analyses of cell lineages within the caudal fin identified pigment cells, fibroblasts, and neural crest-derived Schwann cells running through the rays (Lee et al, 2013;Tu and Johnson, 2011). Additionally, staining of fins with general axonal markers indicates axons innervate the adult fin (König et al, 2019;Lisse et al, 2016;Lee et al, 2013). However, the identity of these axons and the relationship between nerves and other anatomical structures in the caudal fin remains poorly described.…”

“…Samples were blocked in 5% goat serum in PBST (PBS + 0.3% Triton) for at least 1 h at room temperature prior to antibody staining (König et al, 2019), then incubated in primary antibody diluted in PBST overnight at 4°C. The following day, following 4x15 min washes in PBST, samples were incubated with the appropriate secondary antibody diluted in PBST overnight at 4°C.…”