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Highly purified oligodendroglial lineage cells from mice lacking functional bax and bak genes were resistant to apoptosis after in vitro differentiation, indicating an essential role of the intrinsic apoptotic pathway in apoptosis of oligodendrocytes in the absence of neurons (axons) and other glial cells. These mice therefore provide a valuable tool with which to evaluate the significance of the intrinsic apoptotic pathway in regulating the population sizes of oligodendrocytes and oligodendroglial progenitor cells. Quantitative analysis of the optic nerves and the dorsal columns of the spinal cord revealed that the absolute numbers of mature oligodendrocytes immunolabeled for aspartoacylase, and adult glial progenitor cells expressing NG2 chondroitin sulfate proteoglycan, were increased in both white matter tracts of adult bax/bak-deficient mice, and, to a lesser extent, bax-deficient mice, except for no increase in NG2-positive progenitor cells in the dorsal columns of these strains of mutant mice. These increases in mature oligodendrocytes and progenitor cells in bax/bak-deficient mice were unexpectedly proportional to increases in numbers of axons in these white matter tracts, thus retaining the oligodendroglial lineage to axon ratios at most 1.3-fold of the physiological numbers. This contrasts to the prominent expansion in numbers of neural precursor cells in the subventricular zones of these adult mutant mice. Our study indicates that quantitative homeostatic control of the oligodendroglial lineage is distinct from that of neural precursor cells, and that other regulatory mechanism(s), in addition to apoptotic elimination through the intrinsic pathway, prevent the overproduction of highly mitotic oligodendroglial progenitor cells. |
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