The early development of the primary sensory neurones in an amphibian embryo: a scanning electron microscope study.

We have described the development of the primary sensory system of the trunk region of Xenopus laevis embryos from larval stages 21 to 32. The system is based upon Rohon-Beard and extramedullary cells, which have central axons forming a dorsolateral spinal tract and peripheral neurites which innervate the skin. The pioneer axons of the central tract grow along the outer surface of the cord at stage 22. These pioneer axons may be used by secondary axons as a growth substrate. As the tract forms it is covered by the radially expanding distal processes, 'end feet', of the ependymal cells of the cord. Cell bodies of the extramedullary cells bulge out of the cord surface, and are first seen between the newly segmented myotomes, at stage 24. Peripheral neurites from these extramedullary cells grow out laterally from the cord. The Rohon-Beard cells, located within the cord, produce similar peripheral neurites which grow laterally with the extramedullary cell neurites, using them as a substrate. The neurites form bundles which coincide with the intermyotomes and are periodically spaced. The growth cones of these neurites contact the outer surface of the myotomes and proceed ventrally, first on the myotomes and then on the basal lamina of the skin. 'Pioneer' neurites are used by later neurites as a growth substrate, but not to the exclusion of all other substrates. The neurites form a plexus on the skin's basal lamina and contact the underlying epidermal cells through holes in the basal lamina. These holes occur in positions over the intercellular boundaries of the epidermal cells.