Volume 16, Number 21,
Issue of November 1, 1996
pp. 6886-6895
Copyright ©1996 Society for Neuroscience
Density-Dependent Feedback Inhibition of Oligodendrocyte
Precursor Expansion
Received Feb. 13, 1996; revised Aug. 7, 1996; accepted Aug. 9, 1996.
Hong Zhang and
Robert H. Miller
Department of Neuroscience, Case Western Reserve University School
of Medicine, Cleveland, Ohio 44106
The myelin sheath in the vertebrate CNS is formed by
oligodendrocytes. The number of oligodendrocytes in a mature axon tract
must be sufficient to myelinate all appropriate axons. How the number
of oligodendrocytes is matched to axonal requirements and whether such
matching involves axon-oligodendrocyte signaling or intrinsic
oligodendrocyte self-regulation are not clear.
Using a combination of in vitro analyses, we demonstrate
that oligodendrocyte precursors closely regulate their numbers through
interactions between adjacent precursors. In low-density rat spinal
cord cultures, the number of oligodendrocyte lineage cells increases
rapidly. The addition of large numbers of oligodendrocyte precursors
substantially reduces precursor expansion and results in a
normalization of oligodendrocyte lineage cell numbers in the cultures
over time. Thus, the number of oligodendrocyte lineage cells that
develop appears dependent on the density of oligodendrocyte lineage
cells. This normalization of cell number is reflected in assays of
clonal potential and proliferation. For example, precursors gave rise
to fewer progeny and proliferated less at high density. Reduced
precursor expansion at high density was not attributable to the
depletion of growth factors. Cocultures of high and low densities did
not inhibit precursor expansion in low-density cultures, suggesting the
requirement for local cell-cell interactions. The inhibition of
precursor expansion was cell-type-specific and dependent on the
presence of oligodendrocyte lineage cells. We propose that this
density-dependent feedback inhibition of oligodendrocyte precursor
expansion may play a primary role in regulating the number of
oligodendrocytes in the developing spinal cord.
Key words:
spinal cord;
oligodendrocyte precursors;
cell
proliferation;
retroviral analysis;
density dependence