Growth pattern of pioneering chick spinal cord axons.

The early growth pattern of axons in the embryonic chick spinal cord was studied by electron microscopy. Serial perisagittal thin sections were obtained from the lateral margins of spinal cords of stage 17 (S17) and S19 embryos. A simple stereotypic pattern of axonal growth was found. Axons originated from a dispersed population of presumptive interneurons located along the lateral spinal cord margin. They first grew ventrally in a nonfasciculative pattern and later turned at right angles and grew in a fasciculative manner longitudinally in the ventrolateral fasciculus. Growth along the circumferential pathway was analyzed in detail by reconstructing individual axons and growth cones from the S17 specimen. Most circumferential axons, regardless of their site of origin, grew in a parallel orientation, and each of their growth cones projected ventrally. This pattern suggested that circumferential growth cones were guided at many, if not all, points along their path. Study of the region in front of these seven growth cones, however, revealed no apparent structural basis for their guidance. Alternative guidance mechanisms are discussed. In conjunction with previous studies (e.g., Windle and Baxter, 1936; Lyser, 1966), these findings suggest that the circumferential-nonfasciculative and the longitudinal-fasciculative patterns of axonal growth are the two fundamental patterns followed by most early forming axons in the brain stem and spinal cord of all higher vertebrates.