Anatomical Studies of the Medullary Facial Lobe in the Sea Catfish Plotosus lineatus

Abstract

Cell morphology in the paired medullary facial taste nuclei, the facial 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 horseradish peroxidase (HRP).
The FL is extraordinarily developed in the sea catfish, correlating with an enormous number of taste buds on the lips, the adjacent oral mucosa, four pairs of barbels, and over the whole body surface.The FL is divided by fiber fascicles into highly distinct lobules, constituting 5 longitudinal columns through the FL. These lobules begin approximately at the level of posterior one-third of the FL and extend rostrally. Small (7-9 μm) and large-sized (15-20×12-15 μm) neurons are present in the barbel lob-ules. Small neurons form cell clusters in the lobules and large neurons are mainly present in the periph-ery of the lobule. Golgi analyses revealed that the small neurons are possessed of 50-60 μm dendritic fields around them, while the larger ones have thick dendrites of 150 μm in length.
Transganglionic tracing of the peripheral facial rami with HRP showed a distinct somatotopic projection of the rami to the FL. Four lobules (from medal to lateral) have connections with the medialmandibular, lateral mandibular, maxillary, and nasal barbels, respectively. The fifth lobule, located dorsolateral to the other lobules, receives input from the body surface. The upper lip, lower lip and an-teriorpalate behind the upper lip are represented in the lateral, medial and ventral prtions of the posterior one-third of the FL, respectively.
Tracing experiments also showed that the trigeminal fibers contained in each faial ramus project directly through the descending trigeminal tract to the limited region of the FL wherethe facial fibers in the same ramus also project. These results showed the distinct topographical organization in the FL for the trigeminal fibers in registerwith corresponding facial fibers.