The aim of the present study is to define the trajectory of growing axons as well 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 constructed around the tracts of the postoptic commissure (TPOC), a pair of longitudinal tracts that course along the ventrolateral surface of the brain. These tracts are connected across the midline by two commissures. Three short 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 molecule 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 the ventral tegmental commissural pathway in the rostral midbrain. 5. Unique guidance cues are responsible for neuronal pathfinding at specific points in the trajectory of growing axons in the rostral brain. We have shown that selective fasciculation mediates the turning of axons into the major longitudinal tract, while specific cues are required for axons to exit this pathway.
