Axonal growth cones in the developing amphibian spinal cord.

Axonal growth cones in the spinal cord of embryonic and larval Xenopus (stages 24-48) were filled with the anatomical tracer horseradish peroxidase (HRP). Growth cones of lateral and ventral marginal zones, including those of descending spinal and supraspinal pathways, were labeled by application of tracer to the caudal medulla or to one of several levels of the spinal cord. Central axons of sensory neurons were filled via their peripheral processes. Growth cone configuration varied widely but fell into five general categories: complex with both filopodia and veils, filopodia only, lamellipodia only, clavate, and fusiform. Several general patterns emerged from the distribution of these various configurations. Growth cones of younger animals generally were more complex than those of older ones. Growth cones closer to the leading edge of descending fiber bundles were more elaborate than those that followed. Growth cones of the dorsolateral fascicle, which carries ascending central processes of Rohon-Beard and sensory ganglion neurons, were very simple and followed a straight course rostrally, whereas those of descending axons of the lateral fiber areas were more complex and sometimes spread over almost the entire lateral marginal zone. Growth cones of Rohon-Beard central ascending axons were fusiform or clavate, while those of sensory ganglion axons showed several fine filopodia at their tips. Growth cones of both types of sensory axons change configuration as they approached the hindbrain, becoming more complex. This study demonstrates that the configurations of growth cones belonging to the same axonal pathway vary with age and with position along their routes, and that growth cones of different neuron classes exhibit characteristic ranges of morphological variation.