The relation between soma position and fibre trajectory of neurons in the mesencephalic trigeminal nucleus of Xenopus laevis.

In adult Xenopus laevis the mandibular and ophthalmic branches of the trigeminal nerve were backfilled with CoCl2 or cobaltous lysine and whole brains silver intensified to reveal neurons of the mesencephalic Vth nucleus (mes. V). The nucleus contains about 100 cells arranged in a band extending arch-like from the ventrolateral margin of the optic tectum to the midline. Many cells possess a small number of short dendritic processes that arborize in the tectal neuropil; in some cells one dendrite terminates within the ependyma or ventricle. A single axon arises from each cell and courses in layer 7 to the margin of the tectum. Axon collaterals arise close to the cell body to terminate principally within layer 6, but occasionally also in layers 8 and 9. Collaterals occurring more caudally terminate in layer 6. These findings suggest that mes. V cells acts as tectal interneurons as well as conveying somatosensory information to the tectum from the mouth region. In the dorsal roof of the tectum the trajectory of a fibre is related to the distance of the soma from the midline. Mes. V cells located at the lateral end of the nucleus possess axons that course initially in a mediolateral direction before turning along the ventrolateral margin of the tectum. Cells positioned close to the midline have axons that project rostrocaudally the entire length of the tectum. The axons of cells located at intermediate positions within the nucleus course at correspondingly oblique angles through the dorsal roof of the tectum. Thus in this area there is a more or less 90 degrees rang in the orientation of mes. V fibres to the longitudinal axis. It is proposed that this topographical relationship between soma position and axon trajectory arises through a developmental mechanism, in which mes. V fibres grow during larval life sequentially into the medial zone of tectal growth and become subsequently displaced rostrolaterally, owing to the further addition of tectal tissue medially, through an angle dependent upon the parent cell's date of birth.