Two populations of microglial cells isolated from rat primary mixed glial cultures.

Because of variations in the morphology and function of microglial cells, it has often been claimed that microglial cells should be classified into two or more subtypes. However, such subtypes have not fully been characterized. In the present study, we isolated microglial cells expressing microglia-markers CD11b and CD68 from rat mixed glial cultures on the fifth and on the thirteenth days in vitro (DIV 5 and 13) and demonstrate that these two populations of microglial cells have distinct morphology and function. Microglial cells isolated on DIV 5, which we have termed immature cells, are characterized by the presence of large somata, large peroxidase- and alkaline phosphatase-positive granules, and high proliferative activity and suppressed responsiveness to lipopolysaccharide (LPS). In contrast, the microglial cells isolated on DIV 13, which we have termed mature cells, are devoid of granules, appear to be in a state of cell cycle arrest, and respond to LPS by the induction of inducible nitric oxide synthase (iNOS), tumor necrosis factor-alpha, and interleukin-6. Isolated immature cells maintained in pure culture failed to express iNOS in response to LPS. However, if these cells were cultured on astrocyte-derived extracellular matrix (AsECM) or pure laminin, the cells exhibited an induction of iNOS in response to LPS. AsECM and laminin were also able to induce a state of cell cycle arrest in cultured isolated immature cells. Thus, classification into two types of microglial cells is possible, but both types are in the same cell lineage, because the immature cells can differentiate into mature microglial cells in the presence of laminin or AsECM. Therefore, "microglioblasts" may be the appropriate term for the immature cells.