Development of microglia in mouse neopallial cell cultures.

Microglia develop in cultures initiated from disaggregated neopallial cells of newborn C3H/HeJ mice when the cultures are subjected to nutritional deprivation for 10 or more days (Hao et al: Int J Dev Neurosci 9:1-14, 1991). In the present experiments, the cultures were pulsed with BrdU for 3 hours at different times during incubation and then the cells were immunoreacted with antibodies against BrdU, GFAP, and CR3 receptor. The dividing cells (BrdU+) were found to be either GFAP+ or GFAP-, but not Mac-1+/BrdU+. Infection of proliferating cells after 2 or more days of incubation with replication-deficient retroviral vector containing E. coli lacZ reporter gene resulted in many labeled astroglia cell clones but no labeled microglia. However, when cells were infected right after disaggregation of neopallium, labeled Mac-1+ microglia were found. When Mac-1+ cells in a suspension of disaggregated neopallial cells were killed using complement mediated lysis before setting up the cultures, Mac-1+ microglia developed, in spite of the treatment. We conclude that in cultures initiated from mouse neopallium there are MAC-1-/GFAP- microglia progenitor cells which do not divide in nutritionally deprived cultures but can transform into Mac-1+ microglia under the influence of astroglia-derived trophic factors. Microglia, which become Mac-1+ (i.e., express CR3 receptor), proliferate extensively in the presence of CSF-1 (which is produced by astroglia).