Is the outgrowth of transplant-derived axons guided by host astrocytes and myelin loss?

Loss of cortical neurons may lead to sever and sometimes irreversible deficits in motor function in a number of neuropathological conditions. Absence of spontaneous axonal regeneration following trauma in the adult central nervous system (CNS) is attributed to inhibitory factors associated to the CNS white matter and to the non-permissive environment provided by reactive astrocytes that form a physical and biochemical barrier scar. Neural transplantation of embryonic neurons has been widely assessed as a potential approach to overcome the generally limited capacity of the mature CNS to regenerate axons or to generate new neurons in response to cell loss. We have recently shown that embryonic (E14) mouse motor cortical tissue transplanted into the damaged motor cortex of adult mice developed efferent projections to appropriate cortical and subcortical host targets including distant areas such as the spinal cord, with a topographical organization similar to that of intact motor cortex. Several parameters might account for the outgrowth of axonal projections from embryonic neurons within a presumably non-permissive adult brain, among which are astroglial reactions and myelin formation. In the present study, we have examined the role of astrocytes and myelin in the axonal outgrowth of transplanted neurons.