DIVERGENT LINEAGES FOR OLIGODENDROCYTES AND ASTROCYTES ORIGINATING INTHE NEONATAL FOREBRAIN SUBVENTRICULAR ZONE

Although previous studies have revealed that the prenatal rat ventricu lar zone contains separate progenitor cells for neurons, astrocytes, a nd oligodendrocytes during the development of the cerebral cortex as e arly as the beginning of neurogenesis (Luskin et al., 1993; Grove et a l., 1993), it is still unclear whether there are bipotential progenito r cells in the neonatal telencephalic subventricular zone which give r ise to both astrocytes and oligodendrocytes during the peak of gliogen esis. To investigate this possibility, discrete groups of clonally rel ated cells, generated by infecting progenitor cells of the neonatal su bventricular zone with a retroviral lineage tracer, were analyzed ultr astructurally. An intracerebral injection of retrovirus encoding the r eporter gene E. coli beta-galactosidase (lacZ) was made into the subve ntricular zone of newborn rats. Two weeks later their brains were perf used, sectioned, and histochemically reacted with X-Gal to identify at the light microscopic level clones of lacZ-positive cells. The sectio ns were processed for electron microscopy to enable the identity of cl onally related cells to be assessed at the ultrastructural level. All of the clones analyzed contained cells of the same phenotype and could be divided into four distinct types: immature cell clones situated in the subependymal zone surrounding the lateral ventricle, oligodendroc ytes clones, and white or gray matter astrocyte clones. Not all of the cells in every clone displayed ultrastructural features of a mature c ell. Rather, in some glial clones the lacZ-positive cells appeared to be at different stages of differentiation. However, we never encounter ed clones which contained both macroglial subtypes or clones containin g neurons. Although the existence of bipotential progenitor cells cann ot be completely dismissed, our results indicate the absence of progen itor cells in vivo in the neonatal subventricular zone which divide an d generate astrocytes and oligodendrocytes. (C) 1994 Wiley-Liss, Inc.