occurred latter than in white matter. The hippocampus also displayed a later onset of preOL maturation in all three strains, such that OL lineage maturation and early myelination was not observed to occur until about P14. This timing of preOL maturation in rat cortical gray matter coincided with a similar timing in human cerebral cortex, where preOL also predominated until at least 8 months after full-term birth. These studies support that strain-specific differences in OL lineage immaturity were present in the early perinatal period at about P2, and they define a narrow window of preterm equivalence with human that diminishes by P5. Later developmental onset of preOL maturation in both cerebral cortex and hippocampus coincides with an extended window of potential vulnerability of the OL lineage to hypoxiaischemia in these gray matter regions.

Mixed glial primary cultures derived from neonatal rat brain were used to isolate the progenitor glial cell with the capacity to differentiate into oligodendroglia or type 2 astrocytes depending on the culture medium. Subcultures composed primarily of this progenitor were utilized, first, to study the regulation of oligodendroglial differentiation by two factors added to chemically defined medium (CDM), i.e. boiled fetal calf serum (FCS) and cellular extract of type 1 astrocytes, and, second, to devise culture conditions that would cause the progenitor cells to differentiate virtually exclusively along oligodendroglial lines (judged by immunologically identified expression of galactocerebroside; GC) and with high specific and total activity of the oligodendroglial enzymes, 2'',3''-cyclic nucleotide 3''-phosphohydrolase (CNP) and glycerol-3-phosphate dehydrogenase (GPDH). Addition of untreated FCS to CDM of the progenitors resulted in 6 days in richly cellular, mixed glial cultures composed of slightly more GFAP-positive astrocytes than oligodendroglia. Addition of boiled FCS to CDM of the progenitors resulted in 6 days in cultures composed almost exclusively (95%) of GC-positive cells and with high specific activity of CNP. This effect of boiled serum, compared to untreated serum, was related to virtual elimination of astrocytes, in the presence of continued differentiation to GC-positive oligodendroglia. Addition of type 1 astrocyte extract as well as boiled FCS to CDM was necessary to generate high specific activity of GPDH. Additionally, the total number of oligodendroglia increased 2-fold when astrocyte extract as well as boiled FCS was added to the CDM. We conclude that although growth of progenitor cells in both boiled FCS and astrocyte extract results in virtually pure cultures of GC-positive oligodendroglia, these two factors have different and complementary effects. Boiling of FCS appears primarily to remove a stimulatory factor for astrocytes and to preserve a stimulatory heat-stable serum factor for oligodendroglial differentiation, as manifested by GC expression and high CNP-specific activity. Addition of astrocyte extract to boiled FCS stimulates, more than does boiled FCS alone, both oligodendroglial proliferation and differentiation, the latter manifested additionally by GPDH-specific activity. The combination of (1) subcultures of the oligodendroglia - type 2 astrocyte progenitor cell, and (2) a culture medium consisting of CDM supplemented with both boiled FCS and type 1 astrocyte extract constitutes an excellent system for the study of oligodendroglial differentiation.