Microglia orchestrate neuronal activity in brain organoids

Human stem cell-derived brain organoids provide a physiologically relevant in vitro 3D brain model for studies of neurological development that are unique to the human nervous system. Prior studies have reported protocols that support the maturation of microglia from mesodermal progenitors leading to innately developing microglia within the organoids. However, although microglia are known to support neuronal development in rodents, none of the previous studies have reported what is the impact of microglia on neuronal growth and maturation in human brain organoids. Here we show that incorporating microglial progenitors into the developing organoid supports neuronal maturation, the emergence of neurons capable of firing repetitive action potentials and the appearance of synaptic and neuronal bursting activity. Immunocompetent organoids enable experimental strategies for interrogating fundamental questions on microglial and neuronal diversity and function during human brain development.