Remodeling of Optic Nerve Myelin Sheaths and Axons During
Metamorphosis in Xenopus laevis

As amphibian tadpoles mature into frogs, there are a number of metamorphic changes that alter the relationship between the brain and peripheral sense organs.  Using whole mount and light and electron microscopic preparations, we observed that an actual shortening of nerves may occur.  While it was well known that nerves may lengthen during growth, programmed shortening had not been reported previously.  As the optic nerve shortens, the fibers fold back on themselves and form redundant loops; these loops would obviously be a physiological disadvantage because of the unnecessarily long axon route of nerve conduction.  However, we noted that fairly rapidly after formation of the redundant material, the extra folds of axon membrane are pinched off, internalized in the axon, and  the axon length decreases.  The glial cells similarly remove their redundant material and establish an optimal neuron-glial arrangement.  The stimulus that initiates the changes and the mechanisms that signal the appropriate amount and site of membrane removal are not known.