ANATOMICAL STUDIES OF THE MEDULLARY FACIAL LOBE IN THE SEA CATFISH PLOTOSUS-LINEATUS

Cell morphology in the paired medullary facial taste nuclei, the facia l lobe (FL) of the sea catfish Plotosus lineatus was studied in Nissl and Golgi-impregnated materials. The central projections of the facial nerve rami to the FL were examined by transganglionic labeling with h orseradish peroxidase (HRP). The FL is extraordinarily developed in th e sea catfish, correlating with an enormous number of taste buds on th e lips, the adjacent oral mucosa, four pairs of barbels, and over the whole body surface. The FL is divided by fiber fascicles into highly d istinct lobules, constituting 5 longitudinal columns through the FL. T hese lobules begin approximately at the level of posterior one-third o f the FL and extend rostrally. Small (7-9 mu m) and large-sized (15-20 x 12-15 mu m) neurons are present in the barbel lobules. Small neuron s form cell clusters in the lobules and large neurons are mainly prese nt in the periphery of the lobule. Golgi analyses revealed that the sm all neurons are possessed of 50-60 mu m dendritic fields around them, while the larger ones have thick dendrites of 150 mu m in length. Tran sganglionic tracing of the peripheral facial rami with HRP showed a di stinct somatotopic projection of the rami to the FL. Four lobules (fro m medal to lateral) have connections with the medial mandibular, later al mandibular, maxillary, and nasal barbels, respectively. The fifth l obule, located dorsolateral to the other lobules, receives input from the body surface. The upper lip, lower lip and anterior palate behind the upper lip are represented in the lateral, medial and ventral porti ons of the posterior one-third of the FL, respectively. Tracing experi ments also showed that the trigeminal fibers contained in each facial ramus project directly through the descending trigeminal tract to the limited region of the FL where the facial fibers in the same ramus als o project. These results showed the distinct topographical organizatio n in the FL for the trigeminal fibers in register with corresponding f acial fibers.