We have investigated the expression of the highly polysialylated neural cell adhesion molecule in the mouse spinal cord during postnatal myelination and in the adult after chemically induced demyelination. By double immunohistochemistry, using a monoclonal antibody (anti-Men B) which specifically recognizes polysialic acid (PSA) units on neural cell adhesion molecule (N-CAM), and an anti-myelin basic protein, a caudorostral gradient of expression of PSA-NCAM was observed at postnatal day 1 (P1), which was inversely related to the gradient of myelination. At P7, PSA-NCAM labelling decreased relative to P1. In white matter, this decrease was correlated with the progression of myelination. PSA-NCAM immunoreactivity persisted in as yet unmyelinated structures, i.e. the corticospinal tract, the dorsomedial part of the ventral funiculus and the lateral funiculi, and decreased with the onset of myelination of these structures at P15. In the adult, PSA-NCAM expression remained in discrete structures, i.e. laminae I and II of the dorsal horn and lamina X around the central canal. The ependymal cells and the astrocyte endfeet under the meninges were also labelled. In addition, PSA-NCAM expression was reinduced on various cells and structures after lysolecithin-induced demyelination of the adult mouse spinal cord. At early times after demyelination, PSA-NCAM was expressed on glial cells around the lesion but also at a distance from this zone. Seven days after injection, cellular PSA-NCAM expression was found around but also within the lesion. This expression was totally abolished 15 days after injection. Double immunohistochemistry for PSA and cell-specific markers showed that the cells which expressed PSA-NCAM after demyelination were oligodendrocyte precursors, reactive astrocytes and Schwann cells. PSA-NCAM re-expression on all cell types was transient and ceased when myelin repair was accomplished. The spatial and temporal regulation of PSA-NCAM expression during development and after demyelination suggests a role for PSA-NCAM in glial plasticity during the myelination and remyelination processes.

The synthesis of platelet-derived growth factor-α receptor (PDGF-αR) is commonly attributed to oligodendrocyte progenitors during late embryonic and postnatal development. However, we recently demonstrated that mature neurons could also synthesize PDGF-αR, emphasizing a larger role for this receptor than previously described. In the present study, to analyze the pattern of PDGF-αR expression during postnatal development of the mouse CNS, we usedin situhybridization and immunohistochemistry on brain and spinal cord tissue sections. We found that, in addition to immature cells of the oligodendrocyte lineage, neurons of various CNS regions express PDGF-αR transcripts and protein as early as postnatal day 1 (P1). Whereas neuronal expression was maintained at all ages, the oligodendroglial expression strongly decreased after P21. In the adult, PDGF-αR was detected in very few oligodendrocyte progenitors scattered in the cerebral cortex or in white matter tracts, thus suggesting the presence of PDGF-αR on O-2Aadultprogenitors. In the mature CNS, PDGF-αR transcripts and protein were mainly localized in neurons of numerous structures, such as the olfactory bulb, cerebral cortex, hippocampus, and brainstem nuclei and in motor neurons of the ventral horn of the spinal cord. The differential expression of PDGF-αR in oligodendroglia and neurons argues in favor of several roles of PDGF during development.

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