Changes in cell number in the central canal ependyma and in the dorsal grey matter of the rabbit thoracic spinal cord during fetal development.

A quantitative investigation of the fetal rabbit spinal cord (Sturrock, 1982 a) demonstrated that prenatal glial cell production was greatest between 20 days postconception (E20) and birth at 30 days post-conception (E30). The greatest increase in glial cell number occurred in white matter and the major part of this increase appeared to be due to proliferation of glial cells within the white matter. Some authors have suggested that glial cell production in the spinal cord is the result of proliferation of ependymal cells (Sakla, 1965) or of glioblasts situated around the central canal (Fujita, 1965; Gilmore, 1971), but the phase of large scale ependymal proliferation ceased before the period of rapid increase in glial cell number and there was no evidence for the existence of a subependymal layer in the fetal rabbit spinal cord (Sturrock, 1982a). Some mitotic figures were present in the ependyma of the central canal but it was not clear whether the cells produced by such division contributed significantly to the increase in glial cell number. The grey matter dorsal to the mid-point of the central canal was not included in the earlier investigation due to the difficulty of identifying some cells in the dorsal grey matter as either small neurons or astrocytes. Since the earlier investigation was completed practice at identifying dorsal grey matter neurons and glial cells, both in the electron microscope and in 1 gm sections, has enabled the problem of cell identification largely to be overcome and almost all cells in the dorsal grey matter can be identified with confidence from E20 onwards. The present study set out to investigate gliogenesis in the dorsal grey matter, because, if quantitative data of changes in glial cell number in this large area of grey matter were available, it would be possible, by combining these with results of the earlier study, to estimate the relative contribution of cell division in the ependymal layer, grey matter and white matter to prenatal cord gliogenesis. The number of ependymal cells was also estimated because it was not clear from the previous study (Sturrock, 1982a) whether ependymal division contributed significantly to glial cell production or whether it simply gave rise to the increased number of ependymal cells required to line the central canal as it grew in length.