D

Regeneration in the adult spinal cord is impeded by (1) absence of endoneurial tubes that normally guide axon sprouts, (2) limited availability of neurotrophic growth factors, and (3) the formation of a glial scar; in addition, regeneration is blocked by the PRESENCE—not the absence—of myelin-associated inhibitory molecules such as Nogo-A, MAG and OMgp. Therefore, “lack of myelin inhibiting substance” is not a reason for regeneration failure. [Domeniconi & Filbin 2005, PMID 15949495; Fawcett et al. 2012, PMID 23098733]