Morphological differentiation of microglial cells in culture: involvement of insoluble factors derived from astrocytes

It is believed that ramified resting microglial cells in the brain are diff erentiated from macrophage-like ameboid cells, although the mechanism for t he differentiation is not fully understood. In the present study, we invest igated whether the differentiation of microglial cells is observable in mix ed brain cell culture prepared from newborn rat forebrains. In confluent mi xed brain cell culture, both ramified and ameboid microglial cells were sim ultaneously present. The ramified cells were located in or under the astroc yte monolayer, while the ameboid cells were over the layer as revealed by c onfocal laser scan microscopy. The majority of ramified cells appeared afte r the astrocyte layer was completely formed and they downregulated the expr ession of the major histocompatibility complex antigen. Fibronectin was det ected around ramified microglial cells, and laminin was also present in the astrocyte monolayer in mixed brain cell culture, while both proteins were not distributed near ameboid cells over the monolayer. When purified microg lial cells were cultured on astrocyte-derived extracellular matrix in serum -free medium, they ramified. These results show that the differentiation of microglial cells is observable in culture and that astrocytes may play piv otal roles in the differentiation mainly by secreting insoluble factors. (C ) 1999 Elsevier Science Ireland Ltd. All rights reserved.