Differentiation of the human spinal cord and involution of its caudal end were investigated in 4-9-week human conceptuses using immunofluorescence and electron microscopy. In the spinal cord, several types of intermediate filament proteins and desmoglein were expressed in parallel: in early stages (4 to 6 weeks), neurofilaments were expressed in low amounts only in the neuroblast processes of the marginal layer. At 6 weeks, differences in staining intensity and distribution patterns of neurofilaments became apparent between lumbar and sacrococcygeal (tail) parts of the spinal cord. Neurofilament expression increased in the mantle and marginal layers of the lumbar spinal cord coinciding with advancing neurogenesis. In contrast, neurofilament expression decreased in the sacrococcygeal spinal cord in association with regression of all tail organs. Regression was characterized by the appearance of large amounts of dead cells and macrophages. Strong vimentin expression was found in neuroepithelial (ependymal) cells and in the radial glia of the spinal cord throughout all stages examined. Coexpression of vimentin and glial fibrillary acidic protein was found only in the radial glia in the earliest developmental stage. Desmoglein was expressed in low amounts around the central canal which was probably associated with the immature junctional complexes that were present between ependymal cells. In conclusion, temporal and spatial distribution patterns of intermediate filament proteins in specific cell populations characterizes differentiation and caudal involution of the human spinal cord.