Macrophage colony stimulating factor (CSF1) is a primary actor of proliferation, differentiation and function of myeloid cells, but microglia develop normally in CSF1 mutant mice, though their numbers are drastically decreased in CSF1R mutant animals 16. Therefore, it was suggested that an additional cognate ligand of CSF1R is crucial for microglia development. Addition of the recently identified alternative ligand for CSF1R, Interleukin-34 (IL-34) to the base medium improved the morphological phenotype of microglia-like cells. Immature pMGLs delaminate and adopt directly a macrophage identity, with no colony-forming intermediate. This is consistent with our current understanding of microglial development, which supports the notion that microglia derive from non-monocytic primitive myeloid cells acting as precursors, unlike adult bone marrow-derived macrophages, but similarly to some other tissue-resident macrophages 37. As pMGLs populate the substrate, they clearly isolate from their nearest neighbors, an in vitro behavior which may correspond to the ability to tile the CNS by their in vivo counterparts. Unlike other tissue-resident macrophages, microglia derive from PU.1+/IRF8+/MYB− precursors 34,35. Expression of the same markers in pMGLs supports this ontogeny. pMGLs kept in mono-culture
