Large numbers of dying cells are found in proliferating tissues, suggesting a link between cell death and cell division. We detected and quantified dying cells during pre-and early postnatal development of the rat cerebral cortex using in situ end labeling of DNA fragmentation [terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling (TUNEL)] and electron microscopy. The proliferative zones that give rise to the neuronal and glial cell types of the cortex, the ventricular and, to a larger extent, the subventricular zones showed higher incidence of cell death than other regions of the developing cortex during the period of neurogenesis. Gel electrophoresis of DNA isolated from the subventricular zone of newborn animals showed a ladder pattern that is characteristic of apoptosis. The number of apoptotic cells remained high in this zone for at least 2 weeks, during which period cells continued to divide. The correlation between cell division and cell death was studied in the subventricular zone of newborn rats; cumulative labeling with bromodeoxyuridine showed that 71% of TUNEL-labeled cells had taken up this S-phase marker before undergoing cell death. Using bromodeoxyuridine and [ 3 H]-thymidine in succession to identify a cohort of proliferating cells, we found that the clearance time of TUNEL-positive nuclei was 2 hr and 20 min. A comparison between the number of mitotic figures and that of TUNEL-positive nuclei showed that cell death affects one in every 14 cells produced by dividing ventricular zone cells at embryonic day 16 and one in every 1.5 cells produced in the subventricular zone of newborn rats. In addition, we found that most of TUNEL-positive cells were in the G1 phase of their cell cycle. We conclude that apoptosis is prominent in the proliferating neuroepithelium of the developing rat cerebral cortex and that it is related to the progression of the cell cycle.

Schwann cells (SCs) are among the most attractive cellular candidates for the development of remyelination therapies for CNS lesions. Yet, their integration in the CNS is inhibited by astrocytes and therefore the use of genetically modified SCs with improved properties is an alternative promising approach. Our strategy for ameliorating the therapeutic potential of SCs has been to alter their adhesive properties by expressing on their surface the polysialylated (PSA) form of the neural cell adhesion molecule NCAM. In the present study, SCs from transgenic GFP-mice were transduced with a retroviral vector encoding sialyl-transferase X (STX), the enzyme responsible for transferring PSA on NCAM. Engineered STX-GFP-SCs with sustained PSA expression were thus generated and were found to have improved ability to associate with astrocytes in vitro. Importantly, when these cells were transplanted in vivo in a mouse model of spinal cord injury they promoted faster and significantly greater functional recovery as compared to using SCs transduced with a control retroviral vector or no cells at all. Morphological analysis indicated that the improved locomotor recovery correlated with earlier and enhanced remyelination by grafted STX-GFP-SCs, increased remyelination by host SCs as well as enhanced differentiation/remyelination by resident oligodendrocyte precursors. Moreover, sprouting of regenerating serotonergic nerve fibres, which are known to be important for locomotion and recovery after injury, was observed into and across the lesion site. These results underline the potential therapeutic benefit of early activation of myelin-forming cells to differentiate and remyelinate severed axons thus restoring functions in CNS trauma and/or demyelinating diseases.

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Gap junctions are membrane channels that mediate electrical and metabolic coupling between adjacent cells. Immunocytochemical analysis by using a panel of anti-connexin antibodies, as well as electron microscopy of thin sections and freeze-fracture replicas, has shown that gap junctions and their constituent proteins are abundant in the cerebral cortex of the adult rat. Their frequency and distribution vary in different cortical regions, which may reflect differences in the cellular and functional organization of these areas of the cortex. Gap junctions were identified between glial cells and, less frequently, between neuronal elements. Heterologous junctions were also identified between astrocytes and oligodendrocytes and between neurons and glia; the latter category included abundant junctions between astrocytic processes and neurons. Double-antibody labelling experiments in tissue sections and in acutely dissociated cells showed that connexin 32 was expressed in neurons and oligodendrocytes, whereas connexin 43, widely believed to be expressed only in astrocytes, was also localized in a population of cortical neurons. These results show that gap junctions can provide a major nonsynaptic means of communication between cortical cell types.