Observations on the recovering lumbar spinal cord of lizards show multiple origins of the cells forming the bridge region including immune cells

Abstract

After transection the lumbar spinal cord of lizards forms a bridge of connective and nervous tissues between the severed proximal and distal ends of the cord. The types of proliferating cells activated in the injured spinal cord have been analyzed using light and ultrastructural immunolabeling for 5BrdU and nestin from 11 to 34 days after injury, when recovery of some hindlimb movements has occurred. At 11-22 days post-transection an intense proliferation of glial, immune and meningeal cells takes place. Nestin is almost absent in the normal spinal cord but becomes detectable at 11-34 days postinjury in ependymal and sparse glial cells located in the bridge region. At 11-22 days postinjury also numerous macrophages, lymphocytes, and some plasma cells appear proliferating during the intense inflammatory and antimicrobial phase. Phagocytosis within the injured spinal cord probably decreases inflammation and may indirectly promote axonal regeneration. Proliferating cells likely derive from precursor or stem elements of the reactive ependymal epithelium, but also from glial cells and meningeal fibroblasts. This is indicated by the presence of 5BrdU-long retaining labeling cells of glial and fibroblast types located in the stumps of the spinal cord and in the bridge. The present observations suggest that meningeal, ependymal, and numerous glial cells are the precursors of those forming the bridge region. Among glial cells, sparse oligodendrocytes myelinating the few axons present at 34 day after the injury also appear capable to proliferate. The myelinated axons are probably involved in the limited but important functional recovery of limb movements observed after 30-90 days postinjury.