Gliogenesis in the mammalian central nervous system does not cease abruptly like neurogenesis. Instead, glia accumulate over a time period that extends into adulthood. To determine whether new glial cells in the adult cortex arise from resident progenitors and to determine the glial types to which these progenitors give rise to, cells in the perinatal subventricular zone (SVZ) were labeled with replication-deficient retroviral vectors, and clonal clusters of glia in the neocortex were examined from 1 week to 8 months of age. The average clonal cluster size increased during the first month of life. Interestingly, clusters containing oligodendrocyte lineage cells preferentially expanded with age, on average doubling every 3 months. Unexpectedly, the number of cells in astrocyte clusters decreased over time. In heterogeneous clusters, the numbers of oligodendroglia increased, whereas the number of astrocytes did not. Moreover, clonal clusters containing mature glia also contained less mature cells, indicating that clonally related progenitors do not differentiate synchronously in vivo. Thus, progenitors from the SVZ continue to cycle, resulting in an accumulation of oligodendroglia in the neocortex. These slowly cycling cells likely express the NG2 proteoglycan because a subset of the clonal clusters contained NG2(+) cells and these NG2(+) cells accumulated with time.
Copyright 1999 Wiley-Liss, Inc.