Postlarval growth of the peripheral gustatory system in the channel catfish, Ictalurus punctatus

The phenomenon of postlarval cell addition to the peripheral nervous system of fish has been reported for some sensory systems, but has yet to be characterized for the gustatory system. Many fishes, such as catfish, possess taste buds scattered across their body surface, and presumably, the number of taste buds increases during growth of the animal. The present study was undertaken in order to examine the process of growth in the peripheral gustatory system and to determine whether the degree of convergence of receptors onto primary sensory afferents changes during growth. The recurrent facial nerve of channel catfish was used for these studies since this nerve contains no general cutaneous components and innervates taste buds along the fish's body surface. Electron micrographs were made of cross sections of this nerve taken from individuals ranging in size from 5.1 to 39.5 cm standard length. In addition, estimates were made of the number of taste buds innervated by this nerve by determining taste bud density along selected regions of the flank and fins in large and small fish. As catfish get larger, the number of both myelinated and unmyelinated axon profiles in the recurrent facial nerve increases, but at a slower rate than the number of taste buds innervated by this nerve. Thus, on average, the number of taste buds innervated by each fiber increases as the fish enlarges; on average there are 2 taste buds per axons profile in small fish and nearly 14 taste buds per axon profile in large fish. The rate of addition of new axon profiles to the nerve is estimated at roughly 70 per day over the range of sizes studied. Although generation of new ganglion cells and axons may contribute to this increase, several lines of evidence indicate that axonal branching occurs. In addition, the mean axon diameter for both myelinated and unmyelinated axons increases during postlarval growth. The finest myelinated fibers (0.2 micron) in small animals were significantly smaller than the finest myelinated fibers (0.7 micron) in larger animals.