In culture, monocyte-derived macrophages adopt either a “fried-egg” morphology, or an axial bipolar spread reflecting the macrophage polarization and activation status 25. In contrast, a typical characteristic of microglia is to become highly ramified in vivo. To assess the phenotypic characteristics of the cells, we cultured pMGLs in a maintenance medium (MGM) containing high concentrations of IL-34 (100ng/mL) and low concentration of CSF1 (5ng/mL), in NGD base. As shown in Fig. 3a, b and Supplementary Movie 3 and 4, primary human FMGs and pMGLs derived from hES or hiPS cells adopted an identical morphology, characterized by multiple thin first-order branching terminated by actively motile membrane ruffles, around an immobile soma.
