THE DEVELOPMENT OF THE PERIPHERAL NERVOUS SYSTEM IN THE TILAPIA OREOCHROMIS MOSSAMBICUS

Abstract

This study focused on the development of the peripheral nervous system (PNS) in the tilapia fish Oreochromis mossambicus. The gross anatomy of the PNS in the adult fish was determined by simple dissection to acquire the pertinent information on the branching patterns of the spinal nerves and their anatomical relationships with other elements of the PNS, namely the dorsal root ganglion (DRG) and its peripheral fibers. It was found that the pattern of growth of the peripheral nerves are not identical in the bony fish which have been studied. This is an interesting observation since comparative anatomy textbooks ( for example Eaton, 1951; Kent, 1987) claim that the pattern of peripheral innervation in teleosts is comparable to elasmobranchs and higher vertebrates; this report challenges those assumptions and cautions other workers to examine the gross anatomy of the nervous system if novel species of fish are to be studied. Knowledge of the adult anatomy of the PNS allowed for a developmental study to be conducted. The motor nervous system in tilapia is established early (30 hours after fertilization) and is manifest by primary motoneurons; this is a consistent observation among anamniotic vertebrates already studied. Primary motoneurons sprout axons by 40 hours of development, and by 50 hours the ventral ramus is well established and the dorsal ramus begins to occupy the dorsal portion of the somite. Tracing of the spinal rami was facilitated by labeling embyros at fixed ages with the vital dye DiI, and viewing the specimens with a confocal laser scanning microscope. The results show that the pattern of innervation observed in the adult is attained at approximately 80 hours of age postfertilization, prior to the embryo hatching. The ontogeny of the dorsal root ganglion was described using fixed embryos labeled with DiI. Neural crest cells, which differentiate into sensory ganglion cells of the DRG, were observed to commence their migration from an intersegmental position at approximately 42 hours (postfertilization), and travel rostral and caudal to aggregate around the ventral root. Crest cells differentiated into sensory ganglion cells soon after they migrated to their eventual position ganglion (50 hours). cells around The accumulation of the ventral root sensory fibers continued, fibers were and by 80 hours of age, observed to form the sensory afferent dorsal root. The delay between the formation of the motor and sensory components suggests that the primary sensory system (the Rohon-Beard cells and their ascending and descending fibers in the spinal cord) is still functional prior to hatching, and begins to deteriorate with the onset of formation of the DRG and its afferent fibers just before hatching. The ventrally situated dorsal root ganglion found in the tilapia could be unique to teleosts, although similar documentation is lacking (see Romer and Parsons, 1977; evidence that Yokote, 1982). The literature provides the DRG in some fish (e.g., trout) exhibit a position similar to higher vertebrates. The diverse positions of the DRG in different species of fish may reflect different developmental patterns, which could indicate that different mechanisms function in the various species of bony fishes.