Somatotopic organization of the trigeminal ganglion is known in some vertebrates. The precise pattern of somatotopy, however, seems to vary in different vertebrate groups. Furthermore, the somatotopic organization remains to be studied in teleosts. From an evolutionary point of view, the morphology and somatotopic organization of the trigeminal ganglion of a percomorph teleost, Tilapia, were investigated by means of the tract-tracing method using biocytin and three-dimensional reconstruction models with a computer. The trigeminal ganglion was one cell aggregate elongated in the dorsoventral direction, which was separate from the facial and anterior lateral line ganglia. Biocytin applications to the trigeminal nerve root labeled ordinary ganglion cells in the trigeminal ganglion and a few displaced trigeminal ganglion cells in the facial ganglion. Biocytin applications to three primary branches (the ophthalmic, maxillary, and mandibular nerves) revealed that trigeminal ganglion cells were somatotopically distributed in the ganglion reflecting the dorsoventral order of the three branches. Ganglion cells of the ophthalmic nerve were distributed in the dorsal part of the trigeminal ganglion, those of the mandibular nerve in the ventral part, and those of the maxillary nerve in the intermediate part. Some of maxillary and mandibular ganglion cells appear to overlap in their boundary region, whereas ophthalmic ganglion cells did not intermingle with ganglion cells of other branches. Labeled-primary fibers terminated in the sensory trigeminal nucleus, descending trigeminal nucleus, medial funicular nucleus, a ventral part of the facial lobe, reticular formation, and trigeminal motor nucleus. Labeled cells were observed in the mesencephalic trigeminal nucleus and the trigeminal motor nucleus. The results suggest that the morphology and somatotopic organization of the trigeminal ganglion of tilapia are similar to those of mammals, except that the axis of the somatotopic organization of the ganglion in mammals is a mediolateral direction reflecting the mediolateral order of the ophthalmic, maxillary, and mandibular nerves.

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