An ultrastructural examination of early ventral root formation in amphibia.

The morphology of early interactions between neural tube and myotome in the amphibian embryo and tail regenerate was examined using the electron microscope. Two types of contacts were observed. At the most primitive level where the myotome was yet unsegmented, multiple adhesive-type contacts linked neural tube and myotome. In newly segmented areas early ventral roots were recognizable as small bundles of one to five axons extending the short distance to the myotome. There was only one bundle per segment and in addition to axons, each bundle always contained one or more primitive glial cell processes which accompanied axons as they left the cord. At points of root exit primitive glial processes appeared to funnel axons into the root. The cytoarchitecture of the cord and the new roots suggested that the primitive glia may play a role in pathfinding for motor axons as they leave the cord and extend toward their targets.