In the developing spinal cord most oligodendrocyte precursors (OLPs) arise from the ventral ventricular zone (VZ) under the influence of Sonic Hedgehog but a minority is generated from the dorsal VZ in a Hedgehog-independent manner. In the developing forebrain too, OLPs arise from both the ventral and the dorsal VZ. It is not known whether dorsally- and ventrally- derived oligodendrocyte (OL) lineage cells have different properties. We generated a dual reporter mouse line to color code ventrally- and dorsally-derived OLPs (vOLPs and dOLPs) and their differentiated oligodendrocyte progeny (vOLs and dOLs) for functional studies. We found that ~80% of OL lineage cells in the postnatal spinal cord and ~20% in the corpus callosum are ventrally-derived. In both spinal cord and corpus callosum, vOLPs and dOLPs had indistinguishable electrical properties, as did vOLs and dOLs. However, vOLPs and dOLPs had different migration and settling patterns. In the spinal cord, vOLPs appeared early and spread uniformly throughout the cord whereas dOLPs arrived later and remained mainly in the dorsal and dorsolateral funiculi. During adulthood, corticospinal and rubrospinal tracts became myelinated mainly by dOLs, even though vOLs dominated these tracts during early postnatal life. Thus, dOLPs are electrically similar to vOLPs but appear to out-compete them for dorsal axons.

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We have sought to determine (1) if thalamic neurons upregulate the growth associated protein GAP-43 as a response to injury, or if a peripheral nerve graft is required to induce, enhance or sustain such a response, and (2) if thalamic neurons with different regenerative potentials also display different GAP-43 responses. Levels of GAP-43 protein (detected by LM and EM immunohistochemistry) and of GAP-43 mRNA (detected by in situ hybridization) were compared in the thalamus of adult rats between 1 d and 180 d after making a stab lesion or after implanting a peripheral nerve autograft. Stab injury is a sufficient stimulus to cause a transient upregulation in GAP-43 expression by neurons in the thalamus (both around the graft tip and in particular in the thalamic reticular nucleus) in the first week after injury but this response is both prolonged, and enhanced in the presence of a peripheral nerve graft. In addition, we demonstrate directly, by double labelling, that neurons of the thalamic reticular nucleus displaying high levels of the mRNA for GAP-43, have axons regenerating in the distal portion of the graft. These findings lend direct support to the hypothesis that upregulation of the GAP-43 gene is essential for prolonged regenerative axonal growth. We also demonstrate GAP-43 protein in graft Schwann cells and in brain astrocytes close to the site of graft implantation.

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