Endogenous Hedgehog Signaling Promotes Gliogenesis In Adult Rat Spinal Cord After Compressive Injury

Objectives: Neurogenesis has been described in various regions of the CNS throughout life Cellular responses after spinal cord injury include activation of astrocytes, degeneration of neurons and oligodendrocytes, and include activation of astrocytes, and reactions of the ependymal layer and meningeal cells. To explore the quantity, distribution and attribution of endogenous neural cell division and regeneration after spinal cord injury (SCI) in adult rats. And explore the expression of Shh mRNA after SCI in rats.Methods: 30 Wistar rats were divided into SCI and control groups. Double immunostaining were application to explore the quantity, distribution and attribution of newly BrdU-labeled mitotic dividing cells. In situ hybridization method was used to observe the changes of Shh mRNA in 1, 3, 7, 14 and 28days post operatively in each group.Results: We examined the extent of endogenous neural cell division and replacement from 2 day to 1 month after thoracal spinal cord injury in adult rats. Bromodeoxyuridine injections revealed an increase of 80-fold in the number of newly divided cells in the rat spinal cord after injury, with an average of 725,000 BrdU-labeled cells identified per rat in the immediate injury zone. By 1 month after injury, 15% of these new cells expressed mature markers of oligodendrocytes and 12% expressed mature astrocytic markers. Newly born oligodendrocytes were present in zones of injury-induced demyelination and appeared to ensheath or remyelinate host axons. BrdU labeling vastly increases adjacent to and remote from rat spinal cord lesion sites. The proportion of newly divided cells expressing mature oligodendrocyte and astrocyte markers sequentially increased from 7 days to 29 days after SCI. Double-labeling for BrdU and, the neuron-specific markers NeuN showed no co-localization, indicating an absence of spontaneous neuronal differentiation and repopulation after SCI. We have investigated the expression of developmentally prominent molecule, Shh, after spinal cord injury of adult rat where neurogenesis does not occur after injury. Expression of Shh was analysed in the adult rat spinal cord after compression injury by in situ hybridization up to 1 month after injury. The mRNA expression levels of Shh increased in the grey matter and /or white matter and in the ependyma rostral and caudal to the lesion site after injury. Shh mRNA was upregulated 7 days after injury with maximal expression levels in grey and white matter 10mm distal to the lesion site. This high level of expression was maintained up to at least 28d after injury. Shh expression in ependymal cells was less pronounced than in grey or white matter and was restricted to 5mm distal to the lesion site, being narrower in its distribution than its occurrence in grey and white matter.Conclusions: SCI resulted in the recruitment of large numbers of newly mitotic dividing cells to the vicinity of injured the spinal cord. Newly divided cells differentiated into mature oligodendrocyte and astrocyte but did not form lasting neurons after SCI. SCI can induce the expression of Shh mRNA. Shh has been playing important roles of regulating adult neural cell regeneration in SCI. Thus, endogenous neural cell replacement is an extensive spontaneous compensatory response to injury in the spinal cord that contributes to neural repair and is a potential target for therapeutic enhancement, synchronously suggesting that Shh gene reflect to some extent an endogenous self-repair potential by recapitulating some features of development.