Serum influences the differentiation of membrane structure in cultured astrocytes

The membranes of mammalian astrocytic processes apposed to blood vessels or forming the surface of the brain contain high concentrations of a characteristic intramembrane particle aggregate, termed "assemblies." In order to identify developmental processes which contribute to this remarkable regional specialization of membrane structure, we have devised culture conditions which support the differentiation of assemblies in secondary cultures of astrocytes derived from neonatal rat forebrain. We report here that different lots of fetal calf serum vary dramatically in their capacity to support the differentiation of assemblies. Fetal calf serum thus appears to exert two distinct influences on astrocyte development: it promotes the differentiation of type 2 astrocytes from bipotential precursor cells, as shown by others, and it influences the density of assemblies in type 1, flat, GFAP-immunoreactive astrocytes in our secondary cultures. Horse serum and defined media also support the appearance of assemblies in flat, GFAP-immunoreactive astrocytes. The separate effects of serum supplementation upon cell lineage and membrane differentiation have to be carefully considered in studies designed to examine factors influencing astrocytic development in vitro.