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Abstract
Glial progenitor cells of the developing CNS committed to the oligodendrocyte lineage
(OPCs) express the chondroitin sulfate proteoglycan, NG2. A proportion of OPCs fail
to differentiate past the stage at which they express NG2 and the lipid antigen O4
and persist in the adult CNS in a phenotypically immature form. However, the physiological
function of NG2(+) cells in the adult CNS is unknown. Using antibodies against NG2
we show that NG2 is expressed by a distinct cell population in the mature CNS with
the homogeneous antigenic phenotype of oligodendrocyte progenitors. The morphology
of NG2(+) OPCs varies from region to region, reflecting the different structural environments,
but they appear to represent a homogeneous population within any one gray or white
matter region. A study of nine CNS regions showed that NG2(+) OPCs are numerous throughout
the CNS and numbers in the white matter are only 1.5 times that in the gray. Whereas
the ratio of OPCs to myelinating oligodendrocytes in the spinal cord gray and white
matter approximates 1:4, gray matter regions of the forebrain have a 1:1 ratio, a
phenomenon that will have consequences for oligodendrocyte replacement following demyelination.
BrdU incorporation experiments showed that NG2(+) cells are the major dividing cell
population of the adult rat CNS. Since very little apoptosis was detected and BrdU
became increasingly present in oligodendrocytes after a 10-day pulse chase, with a
concomitant decrease in NG2(+) BrdU incorporating cells, we suggest that the size
of the oligodendrocyte population may actually increase during adult life.