Med. Sci. 532

DEGENERATION / REGENERATION
Regeneration After a focal transection of axons in the mammalian CNS, the proximal cut end of the axon, which is still connected to the cell body, may attempt to regrow by sending out sprouts. But axonal regeneration is limited; in practice functional recovery has usually been meager.  However, a similar lesion in the mammalian PNS often results in many of those sprouts from the proximal cut end of the axon regrowing to and reinnervating peripheral targets.	Intact	Degeneration	Regeneration
The regenerated axons may become myelinated, too.  Characteristically, the population of regenerated, myelinated axons is smaller in diameter and more numerous than in an undamaged nerve.  That comparison is shown in the two cross-sectons of peripheral nerves in which myelin has been darkened using osmium tetroxide.  The nerve on the right has regenerated and remyelinated axons; the one on the left always remained intact.
In a peripheral nerve with many fascicles (i.e. separate bundles of axons ensheathed in perineurium), good functional recovery is more likely when the lesion does not disrupt the perineurium and epineurium. Those layers are shown in the photomicrograph of a normal nerve at right.  If those layers are disrupted - for example if the nerve is severed - recovery is better if the fascicles proximal and distal to the cut are correctly aligned when the nerve is repaired surgically.
	Neurocytology

Med. Sci. 532