“…The number of MNs that die as a result of axotomy, limb amputation, and nerve crush is diminished as the organism ages, although it is unclear whether MNs become trophic-independent or, if with age, they come to depend on alternative sources of trophic support (Snider and Thanedar, 1989;Crews and Wigston, 1990). Numerous other putative trophic sources exist, including glia (e.g., Schwann cells and astrocytes), afferents, and components of the extracellular matrix (Okado and Oppenheim, 1984;Eagleson et al, 1985;Furber et al, 1987;Yin et al, 1994;Riethmacher et al, 1997;Arce et al, 1998;Wong et al, 1999).…”