Microglial ontogeny and in vitro reconstruction: Bridging development and modeling

Microglia are the resident immune cells of the central nervous system (CNS) with a unique developmental origin distinct from other tissue macrophages. They arise from progenitors generated in the yolk sac during early embryogenesis, subsequently colonize the developing brain, and persist throughout life via self-renewal. Following brain entry, microglial progenitors undergo stepwise differentiation and maturation under the influence of the local microenvironment, giving rise to the transcriptional and functional diversity observed in mature microglia. In recent years, substantial progress has been made in elucidating the molecular and developmental mechanisms governing microglial ontogeny, from early progenitor specification to brain colonization and maturation. In parallel, in vitro approaches for generating microglia have advanced rapidly. These include the use of immortalized cell lines, primary microglial cultures, and, more recently, microglia-like cells derived from human induced pluripotent stem cells (iPSCs), which are increasingly used for disease modeling. While these systems do not fully recapitulate the in vivo environment, they provide powerful and complementary platforms for dissecting specific aspects of microglial biology. Recent efforts to incorporate microglia into transplantation and brain organoid models have enhanced their physiological relevance and applicability to human systems. In this review, we integrate current knowledge of microglial development with in vitro models and discuss how these insights can be leveraged to further refine experimental systems and advance human disease research.