Rj. Colello et al., THE ROLE OF OLIGODENDROCYTES AND MYELIN ON AXON MATURATION IN THE DEVELOPING RAT RETINOFUGAL PATHWAY, The Journal of neuroscience, 14(5), 1994, pp. 2594-2605
In neonatal mammals, newly grown optic axons are uniformly small in di
ameter. In the adult, in contrast, axons within the optic nerve can be
classified into distinct groups according to their diameter. Because
axon diameters are also related to the thickness of the myelin sheath,
which in turn determines the velocity of action potential propagation
, the question of what determines the axon diameter is of critical imp
ortance. In a project aimed at determining the influence of the enshea
thing cell on axon maturation, oligodendrocyte development was prevent
ed by eliminating their precursors by unilateral x-irradiation at birt
h. Axon diameters in both the normal and the myelin-free optic nerves
were then measured at varying stages of development. The results demon
strate that axon diameter growth remained substantially reduced in the
absence of oligodendrocytes. Interestingly, by x-irradiating the opti
c nerve and tract on one side of the brain, fibers crossing the chiasm
became larger as they went from an unmyelinated nerve to a myelinated
tract; fibers on the nonirradiated side became smaller as they went f
rom a myelinated nerve and crossed into the nonmyelinated tract. These
results clearly point to a local regulation of axon diameter by oligo
dendrocytes. Moreover, ganglion cells measured 9 d after the initiatio
n of myelination (postnatal day 6, P6) were of similar size within nor
mal retinas and retinas whose axons were x-irradiated, suggesting that
ganglion cell growth occurs in spite of the lack of myelin and axon d
iameter maturation. Finally, we showed, through both section staining
with antibodies to myelin basic protein (MBP) and Northern blot analys
is using a probe to MBP, that the x-irradiated nerve began a delayed m
yelination period (in a gradient from chiasm to eye) at P15 and reache
d an almost normal myelin pattern at P28. Axons from these nerves grew
to seemingly normal diameter concomitant with this delayed myelinatio
n.