Axon-glial relations during regeneration of axons in the adult rat anterior medullary velum

The anterior medullary velum (AMV) of adult Wistar rats was lesioned in the midsagittal plane, transecting all decussating axons including those of th e central projection of the IVth nerve. At selected times up to 200 days af ter transection, the degenerative and regenerative responses of axons and g lia were analyzed using transmission and scanning electron microscopy and i mmunohistochemistry. In particular, both the capacity of oligodendrocytes t o remyelinate regenerated fibers and the stability of the CNS/PNS junctiona l zone of the IVth nerve rootlet were documented. Transected central AMV ax ons exhibited four patterns of fiber regeneration in which fibers grew: ros trocaudally in the reactive paralesion neuropil (Group 1); randomly within the AMV (Group 2); into the ipsilateral IVth nerve rootlet, after turning a t the lesion edge and growing recurrently through the old degenerated contr alateral central trochlear nerve trajectory (Group 3); and ectopically thro ugh paralesion tears in the ependyma onto the surface of the IVth ventricle (Group 4). Group 1-3 axons regenerated unperturbed through degenerating ce ntral myelin, reactive astrocytes, oligodendrocytes, microglia, and large a ccumulations of hematogenous macrophages. Only Group 3 axons survived long term in significant numbers, and all became myelinated by oligodendrocytes, ultimately establishing thin sheaths with relatively normal nodal gaps and intersegmental myelin sheath lenghts. Schwann cells at the CNS/PNS junctio n of the IVth nerve rootlet did not invade the CNS, but astrocyte processes grew across the junction into the PNS portion of the IVth nerve. The basal lamina of the junctional glia limitans remained stable throughout the expe rimental period.