Otros datos sobre la mejora del interés y la motivación del

The observation that OLPs appear to be generated in the same part of the ventral VZ as somatic MNs (see above) suggests that the same set o f neuroepithelial precursors might first generate MNs, then switch to production of OLPs. This neuron-glia switch might be part o f an intrinsic developmental program in the neuroepithelial precursors. Alternatively, the switch might be triggered by feedback signals from the newly-formed MNs. To test the latter idea, we looked to see if OLs can develop in Isll(-/-) spinal cord, which lack all classes of MNs and at least one class of ventral (VI) INs (Pfaff et al., 1996). Because the Isll(-/-) mice die in utero before the emergence of the OL Lineage, we studied OL development in spinal cord explant cultures

Explant cultures from the spinal cords of E9.5 embryos were established. The embryos were genotyped retrospectively by PCR and Isll/2 immunolabeling as described in 2.3.7. After culturing until the equivalent of the day of birth (11 days in culture), the explants were immunolabeled with anti-galactocerebroside (GC) or anti-myelin basic protein (MBP) to visualize differentiated OLs. In five independent experiments on five

separate litters, we examined a total of 11 Isll (-/-), 26 Isll(+/-) and 6 (+/+) embryos. OLs developed in explants from every one o f these embryos (Fig. 5.9). We conclude that signals from Isll-expressing MNs are unlikely to be required for normal development of OL lineage cells in the spinal cord.

Discussion

Neuroepithelial precursors in the ventral VZ of the spinal cord generate MNs and INs, and then a subset o f precursors starts to produce OLPs. I have mapped the site of origin of OLPs in the spinal cord and found that OLPs originate in the ventral-most part of the Poxd-positive VZ, which at earlier times generates somatic MNs. In Pax6-deficient Small eye mice, the origin o f OLPs is shifted dorsally and both OLPs and Isl2/Lim3 MNs are delayed. I suggest that somatic MNs and OLPs are generated sequentially from a common set of precursors whose position in the VZ is influenced by Pax6.

I. Motor neurons and oligodendrocyte progenitors may decend fro m the same neuroepithelial precursors in a cell-intrinsic program

Development of ventral neurons and OLPs depends on Shh signaling in vivo and both MNs and OLPs are induced by the same concentrations o f Shh in vitro (Pringle et al., 1996). This co-dependence on Shh, together with other evidence (see below), led us to suggest (Pringle et al., 1996; Richardson et al., 1997) that MNs and OLs might descend from a common group o f neuroepithelial MN-OL precursors, specification of which is dependent on Shh. If so, we expect that MNs and OLPs should originate from the same site(s) within the VZ. In support of this idea, I have established that OLPs arise in the ventral-most region of the Poxd-positive domain of the spinal cord VZ, the same region that at earlier times is thought to give rise to somatic MNs of the MMC (Ericson et al.,

In Sey/Sey mice, which lack functional Pax6 protein, somatic MNs and some ventral INs in the spinal cord were found to be reduced in number, interpreted as partial respecification (Burrill et al., 1997; Ericson et al., 1997). To confirm and extend previous studies of neuronogenesis, I followed the expession o f neuronal markers, Isll, Isl2 and Lim3, in the cervical spinal cord. I found a delay in onset of expression of Isl2 and Lim3. Onset o f Isll expression occurred at the normal time, at around E9.5, although at E l 0.5 there were fewer /y/7-positive cells in Sey/Sey embryos than their wild-type littermates. By E l 3.5, the intensities and patterns of expression of all the LIM transcripts were indistinguishable in Sey/Sey and wild-type spinal cords. Therefore, it appears that development o f ventral neurons is retarded, but not blocked, by the loss of Pax6.

If, as 1 suggest, OLPs and MNs share a common hneage, one would predict that OLP development should also be disturbed in Sey/Sey mice. 1 tested this prediction and found that apperance o f OLPs is delayed by about a day in the cervical spinal cord in Sey/Sey, but that they subsequently proliferate and catch up with their wild-type littermates before birth. The fact that both MNs and OLPs originate from the same part o f the VZ, and development of both MNs and OLPs is retarded in Sey/Sey spinal cord is compatible with a common MN-OL lineage but does not exclude other models. For example, there could be a separate pool of dedicated OLP precursors whose maturation depends on signals generated by previously formed MNs. However, our finding that OLs develop normally in explants o f Isll(-/-) spinal cords, in the absence of MNs, tends to argue against a central role for MN-derived signals. It still leaves open the possibility that feedback from other classes of ventral neurons such as the Lim3/Chxl0 (V2) INs might be required. 1 think this is unlikely because normal numbers of ChxlO INs accumulate in the Sey/Sey spinal cord before E l2, prior to the normal onset o f OLP development (Burrill et al 1997). A lineage relationship between MNs and OLs was previously suggested by retroviral clonal analysis in the embryonic chick spinal cord (Leber et al., 1990; Leber and Sanes, 1995). Common neuron-oligodendrocyte precursors have also been identified in cell cultures established from developing cerebral cortex (Davis and Temple, 1994; Qian et al., 1997; Williams et al., 1991). Moreover,

common neuron-glial precursors are a well-established feature o f invertebrate nervous systems (e.g. Udolph, et. al., 1993).