OLIGODENDROCYTE PRECURSOR QUANTITATION AND LOCALIZATION IN PERINATAL BRAIN USING A RETROSPECTIVE BIOASSAY

Several late stages of the oligodendrocyte (OL) developmental lineage can be identified immunologically in the newborn rat brain. However, O L lineage-specific markers are not available for the detection of the less mature, yet determined, OL precursors. We have developed a retros pective bioassay, combining limiting dilution analysis with a novel cu lture system, that quantitatively assesses the developmental potential in vivo of phenotypically undefined OL precursors in order to (1) dem onstrate their existence, (2) estimate their total number in the premy elinated rat brain, and (3) demonstrate their presence in regions dist al to germinal zones at times previously predicted to be devoid of suc h cells. Between embryonic day (E) 21 and postnatal day (P) 0, cells d etermined to become oligodendrocytes increase in frequency similar to 5-fold in the whole brain (from one precursor for every 365 cells to 1 in 74), and similar to 2.5-fold in the telencephalon (from 1 in 298 t o 1 in 115). From these data it is calculated that a pool of similar t o 10(6) phenotypically undefined cells are present in the newborn brai n that are able to differentiate into OL in vitro. Further, by applyin g this assay to tissue samples of subdomains of the developing cerebel lum, we have demonstrated that such cells are present in large numbers as early as E20 in regions sparsely populated with cells expressing t he blastic neural cell marker ganglioside G(D3), suggesting that they migrated to this position as a pre-G(D3)-expressing cell. These result s significantly change the predicted ontogeny of the oligodendrocyte l ineage and should fuel the ongoing search for these early OL precursor s.