Microglia are CNS resident macrophages, and they play important roles in neural development and function. Recent studies have suggested that murine microglia arise from a single source, the yolk sac (YS), yet these studies lack spatial resolution to define the bona fide source(s) for microglia. Here, using light-induced high temporal-spatial resolution fate mapping, we challenge this single-source view by showing that microglia in zebrafish arise from multiple sources. The embryonic/larval microglia originate from the rostral blood island (RBI) region, the equivalent of mouse YS for myelopoiesis, whereas the adult microglia arise from the ventral wall of dorsal aorta (VDA) region, a tissue also producing definitive hematopoiesis in mouse. We further show that the VDA-region-derived microglia are Runx1 dependent, but cMyb independent, and developmentally regulated differently from the RBI region-derived microglia. Our study establishes a new paradigm for investigating the development and function of distinct microglia populations.

Microglia are CNS-resident macrophages and play important roles in neural development and function. However, how microglial precursors born in peripheral tissues colonize the CNS remains undefined. Using in vivo imaging and genetic manipulation of zebrafish, we showed that microglial precursors enter the optic tectum of the midbrain, where the majority of microglia reside during early development, via the lateral periphery between the eyes and brain and the ventral periphery of the brain in a circulation-independent manner. The colonization of the optic tectum by microglial precursors is dynamic and driven by apoptotic neuronal death, which occurs naturally in the midbrain during neurogenesis. We further show that lysophosphatidylcholine, a phospholipid known to be released from apoptotic cells, can promote microglial precursor entry into the brain via its cognate receptors grp132b. Our study reveals that microglia colonization of developing zebrafish midbrain is triggered by apoptotic neuronal death, possibly via releasing lysophosphatidylcholine.

Our understanding on the function of microglia has been revolutionized in the recent 20 years. However, the process of maintaining microglia homeostasis has not been fully understood. In this study, we dissected the features of spinal microglia repopulation following an acute partial depletion. By injecting intrathecally Mac-1-saporin, a microglia selective immunotoxin, we ablated 50% microglia in the spinal cord of naive mice. Spinal microglia repopulated rapidly and local homeostasis was re-established within 14 days post-depletion. Mac-1-saporin treatment resulted in microglia cell proliferation and circulating monocyte infiltration. The latter is indeed part of an acute, transient inflammatory reaction that follows cell depletion, and was characterized by an increase in the expression of inflammatory molecules and by the breakdown of the blood spinal cord barrier. During this period, microglia formed cell clusters and exhibited a M1-like phenotype. MCP-1/CCR2 signaling was essential in promoting this depletion associated spinal inflammatory reaction. Interestingly, ruling out MCP-1-mediated secondary inflammation, including blocking recruitment of monocyte-derived microglia, did not affect depletion-triggered microglia repopulation. Our results also demonstrated that newly generated microglia kept their responsiveness to peripheral nerve injury and their contribution to injury-associated neuropathic pain was not significantly altered.Although neurons in the central nervous system (CNS) have limited capacity for regeneration, glial cells exhibit remarkable self-renewal potential. Aroused from yolk sac progenitors that populate the CNS during embryogenesis, microglia in adulthood has been well recognized for their capability in preserving local homeostasis. Failure to keep up microglia in their normal physiological states leads to alteration in the stability of CNS micro-environment, as microglia are not only overseers of pathological disturbances 1,2 they also have physiological roles in normal CNS function 3,4 . However, the question of how microglia strive to maintain the integrity of the cell population is intriguing and unresolved, it has drawn much attention in recent research of microglia cell biology. Several research groups have investigated microglia repopulation after depletion in the brain parenchyma using genetic and/or pharmacological approaches. The main findings have identified the CNS resident microglia as the cell population responsible for re-establishing the CNS microglia compartment. Elmore et al. 5 reported that following depletion by blocking colony-stimulating factor1 receptor (CSF1R) signaling, microglia can repopulate solely through proliferation of nestin-positive, resident cells which then differentiate into microglia. The notion that microglia repopulation relies fully on CNS resident cells is further supported by the group of Bruttger 6 where Cx3cr1CreER :iDTR system has been used to ablate microglia cells. The participation of bone marrow-derived cells in the regeneration process...