Dendritic branching and growth analysis of an embryonic zebrafish sensory neuron: The Rohon-Beard cell

The primary sensory nervous system of the embryonic zebrafish is composed of a network of sensory cells, interneurons, and motor neurons. This system develops quite rapidly over a period of several hours and becomes functional very early during its formation. Rohon-Beard cells, located dorsally in the spinal cord of the embryo, act as the mechanosensory component of this network, transmitting touch information from the skin to the spinal cord. Each of these cells projects rostral and caudal central axons, and extends a peripheral neurite that branches extensively under the skin of the embryo. Effective and even innervation of the embryonic skin is presumably accomplished by controlled peripheral neurite outgrowth and branching events, which prevent the appearance of over or under-innervated regions.; This study quantitatively documented the development of the Rohon-Beard cell peripheral arbors and highlighted key features during arbor formation. Rohon-Beard cells were fluorescently labeled and imaged using a confocal microscope. Images of Rohon-Beard cell peripheral arbors were analyzed using a variety of quantitative methods. The results of this study suggest that two distinct types of branches are found on the arbors, those formed via growth cone bifurcation and those sprouted interstitially from existing neurites. Also, this work led to the construction of a simple model of Rohon-Beard cell peripheral arbor formation in which the development of the neuritic trees occurs in two stages: an early stage where neurites grow for long distances and bifurcate at their tips and a later stage in which existing neurites sprout shorter branches interstitially along their lengths. The early stage allows for maximal coverage of the uninnervated space under the skin of the embryo and assures an early functionality of the sensory system. The later stage fills in the gaps (uninnervated space) left behind by the passing growth cones of the first stage, resulting in the skin being evenly innervated and presumably increasing the sensitivity and gain of the zebrafish embryo's mechanosensory nervous system.