Neuronal pathfinding during development of the rostral brain in Xenopus

1, The aim of the present study is to define the trajectory of growing axon s as wed as to characterize guidance cues mediating neuronal pathfinding in the rostral brain of embryonic Xenopus, 2, The early embryonic Xenopus brain consists of a stereotypical pattern of axon tracts arranged about orthogonal axes, This scaffold of axons is cons tructed around the tracts of the postoptic commissure (TPOC), a pair of lon gitudinal tracts that course along the ventrolateral surface of the brain. These tracts are connected across the midline by two commissures, Three sho rt tracts join the dorsal brain to the TPOC, 3, The TPOC consists of chemically distinct subpopulations of axons defined by the expression of NOC, novel glycoforms of the neural cell adhesion mol ecule N-CAM. Axons expressing NOC-2 sort out in the ventral portion of the TPOC, NOC-2+ axons in the supraoptic tract appear to fasciculate with axons expressing the same cell adhesion molecule in the ventral TPOC, 4, Chondroitin sulphates modulate the growth of axons from the TPOC into th e ventral tegmental commissural pathway in the rostral midbrain. 5, Unique guidance cues are responsible for neuronal pathfinding at specifi c points in the trajectory of growing axons in the rostral brain. We have s hown that selective fasciculation mediates the turning of axons into the ma jor longitudinal tract, while specific cues are required for axons to exit this pathway.