Mass
Larval zebrafish provide unique access to a whole, intact vertebrate nervous system in a living organism. Whole brain and spinal circuit exploration has been made possible by the optical clarity and extensive genetic toolkit available in the zebrafish. These methodological advantages can also be harnessed to explore the brain and spinal cord function in neuromechanical systems. Here we use larval zebrafish to examine the neural architecture of the pectoral fins, forelimb homologs. Previous work has described motor innervation of intrinsic pectoral fin muscles and the associated kinematics. At the larval stage, the fins beat rhythmically and have been shown to mix fluids for cutaneous respiration. Here, we investigate sensory architecture of the pectoral fin in 5 day post fertilization (dpf) larvae. We look at both Rohon-Beard neurons (RBs). The RBs are an early population of mechanosensory cells considered transient and lost during the juvenile stage. We stochastically labeled RBs by injecting UAS:ptagRFP into isl2b:Gal4 embryos, imaged these at 5dpf with confocal microscopy, and reconstructed neurons and their peripheral arborizations. We found that a subpopulation of islet2B+ RBs located at the level of the fourth and fifth myomeres innervate the pectoral fin. These cells display classic RB morphology with dense primary afferent arborization. Once in the fin, the processes branch and spread through the skin. The degree of innervation of the fin varied between RBs, with some cells branching to cover a large area of the fin while others projected into smaller regions only at the base of the fin. In all cases, RBs consistently had enlargements at the ends of primary afferents within the fin. Taken together, these data provide a morphological description of the sensory innervation of the 5dpf fin by RBs, often associated with early larval behaviors and implicated in somatosensory responses. The sensory map presented here provides a basis for future functional studies of sensorimotor integration in the pectoral fin.