Conclusions

Abstract

Tissue culture studies have served to delineate more precisely the relative importance of components of the microenvironment in the regenerative response. With respect to the hypotheses which seem more relevant to the limited regenerative response in the mammalian CNS (see Sect. 2.7), results from in vitro studies support the in vivo work in that the somal response, the presence for specific growth factors, compatibility between neuronal and non-neuronal cells, non-neuronal cell proliferation and vascular permeability may be of significance. For example, NTFs and NPFs accumulate at CNS wound sites (some of which may be glial-derived), but central neuronal responsiveness is reported to decline with age. Further, the types of glial cells at the lesion site may be important since neurite outgrowth is promoted by contact with glial cells, but not with non-glial cells. Therefore, although not specifically studied, it seems that an ependymal-mesenchymal interaction may not be needed for regeneration by CNS neurons. Restricted non-neuronal cell proliferation may also be beneficial since an AraC-mediated control of non-neuronal cell division, particularly of fibroblastic reactive microglial cells, facilitates enhanced neurite outgrowth. With respect to vascular permeability, the degeneration of neurites in the absence of serum suggests that blood-derived proteins may be needed for the maintenance of axonal regenerative growth in the mammalian CNS.