Type 1 astrocytes inhibit myelination by adult rat oligodendrocytes in vitro

We have determined the effect of Type 1 astrocytes on the myelination of dorsal root ganglion cell axons by oligodendrocytes obtained from adult animals. Experiments were initiated by the addition of oligodendrocytes [purified either by density gradient centrifugation and treatment in culture with 5-fluorodeoxyuridine (FdU) or by fluorescence-activated cell sorting after immunostaining with antigalactocerebroside antibody] to cultures of purified neurons. In control conditions, the added oligodendrocytes proliferate and, after 4 weeks, accomplish substantial myelination of the sensory axons. Type 1 astrocytes (purified from cultures of dissociated newborn rat cerebral cortex by vigorous shaking to remove less adherent cells) or fibroblasts (purified from cultures of cranial periosteum by serial replating) were added to some of these cultures after the oligodendrocytes had attached and started to proliferate. We observed that the added Type 1 astrocytes, but not the added fibroblasts, strongly inhibited myelination and caused decreased oligodendrocyte proliferation or survival. These effects of added Type 1 astrocytes were reproduced with Type 1 astrocyte-conditioned medium. We conclude that Type 1 astrocytes can release soluble factors that inhibit oligodendrocyte myelination.