Identification of location and timing of guidance cues in sympathetic preganglionic axons of the chick.

The outgrowth of preganglionic axons into the sympathetic trunk of the chick occurs at stages 26-27 (5-5 1/2 d), beginning with C16 and T1 and followed by progressively more caudal spinal cord segments. The direction of preganglionic projections is segment specific, with T1 preganglionic neurons projecting predominantly in the rostral direction and T4 preganglionic neurons projecting predominantly in the caudal direction. These projections are correct from the outset. A previous study (Yip, 1987) has shown that removal of the neural crest, precursors of target ganglion cells, did not alter the trajectories of preganglionic axons, indicating that target cues are not required for the guidance of these axons. The present work extends that study and examines, through surgical manipulations of the developing embryo, the location and time of appearance of preganglionic axonal guidance cues. Spinal cord transplantation experiments show that segment-specific patterns of preganglionic outgrowth are determined by the local environment of axonal growth and not by the identity of preganglionic populations. In addition, spinal cord transplants of age-mismatched donors and hosts indicate that the timing of preganglionic outgrowth is also under local environmental control. These findings suggest that preganglionic axonal outgrowth is guided by local cues which may appear in a controlled spatiotemporal manner.