The role of ependyma in regeneration of the spinal cord in the urodele amphibian tail

The new spinal cord formed during tail regeneration in the newt first develops as a caudal extension of the ependymal tube. Neuroblasts and neuroglia subsequently differentiate from cells of the ependymal tube in a proximal-caudal sweep. Descending axons from the cord rostral to the lesion and from newly differentiating neurons travel in channels which are present prior to the ingrowth of axons. The present study confirms previous observations from our laboratory and presents details of the ultrastructural relations of axons and ependymal processes within the cord. The ependymal cell surface facing channels typically forms numerous digitor sheet-like protuberances which extend into the channel lumen. As axons enter the channels in increasing numbers these protuberances partially subdivide the axons into smaller groupings, even occasionally segregating individual axons. At levels where fibers have not yet entered or have most recently entered the ependymal channesl two specializations appear on the ependymal plasmalemma facing the channels and their axons: coated membranes and hemidesmosome-like structures. At more mature levels, where many fibers have already entered the channels, axons in contact with ependymal processes sometimes show synapse-lide vesicle accumulations with associated membrane densities. Coated membranes and hemidesmosome-like structures are lacking at these levels. Our observations suggest that ependymal processes, in addition to providing substrate and direction for regenerating spinal cord axons, may also furnish or exchange more specific information at the morphologically identifiable specializations described above.