The axon determines whether or not it is myelinated by the Schwann
cell.
At maturity there is a positive
correlation between sheath thickness and axon calibre. This correlation
is initially very low or absent, but
gradually strengthens during development. This increase could come about
because the axon continuously
controls Schwann cell myelinating activity, so that a given axon calibre
is associated with a particular myelin
sheath thickness, an interaction which would entail the Schwann cell
continuously monitoring and
responding to axon size. This seems unnecessarily complex. This
theoretical study shows that the strong
correlation between the 2 parameters within a given myelinated fibre
population may come about in a much
simpler way than outlined above. This is demonstrated by modelling
the growth and myelination of a
hypothetical population, utilising data from earlier studies on
cervical ventral motoneuron axon
development. The hypothesis tested shows that the only instructive
interactions by the axon on the Schwann
cell necessary for the strong correlation between the 2 parameters
to emerge are for the initiation of
myelination, its continuation and its termination. These could result
from a single stimulus being switched
on, persisting for a time and being switched off. Under this influence,
the Schwann cell is assumed to
proceed to form the myelin sheath at a constant rate which it itself
inherently determines, in the absence of
any quantitative influence exerted by the axon. This continues until
the stimulus for myelination ceases to
emanate from the axon. The validity of the hypothesis is demonstrated,
because the resulting myelin-axon
relationships correspond closely to those observed during development.