The developing brain is punctuated with critical periods for specific organizational and remodeling events that are guided by sensitivity to endogenous and exogenous stimuli. Perturbations in those stimuli create nodes of vulnerability for dysregulation with enduring consequences and increased risk for neurodevelopmental disorders. Identifying previously unknown critical periods and the parameters that define them are essential steps in the quest to reduce or ameliorate the impact of neurodevelopmental disorders and improve the health of children. Two pervasive features are the heightened risk to males and the deleterious consequences of neuroinflammation as the brain grows. Rarely, however, are these two features considered together. We have previously described roles for the innate immune cells of the brain, microglia and mast cells, in guiding synaptic development in the preoptic area/hypothalamus during a critical period for hormone-mediated sexual differentiation of the brain. Mast cells, unlike microglia, are not unique to the brain but instead occupy a wide variety of niches throughout the body where they subserve numerous functions, many of which are still being discovered and include surprising roles as tissue remodelers. Mast cells in the brain remain poorly understood, particularly during development. The potential for distinct temporally regulated populations within the brain has never been considered. We propose that possibility here and hypothesize there are two distinct populations of mast cells in the developing brain with common and divergent functions in health and disease. These populations are found in the POA/hypothalamus and hippocampus and share the common feature of peaking in density across the first week of life and then precipitously declining. The tightly regulated peak and decline in mast cells is reminiscent of a critical period, which has been established for the POA/hypothalamus. However, the two populations diverge along several lines, the most striking being the high proliferative capacity of the hippocampal mast cells with a complete absence of cell division in the POA/hypothalamic population. There is also a pronounced sex difference in the POA/hypothalamus with males having more mast cells than females but there is no evidence to-date of the same in the hippocampus. We seek to understand the tissue origins, proliferative capacity and functions of both populations with three independent but inter-related specific aims. Aim 1 will test whether mast cells originate in the canonical source, the bone marrow, versus the yolk sac versus a newly identified hematopoietic niche in the skull. Aim 2 will test the hypothesis that c-kit (stem cell factor) regulates the differential capacity of mast cells in the POA/hypothalamus versus hippocampus. Additional candidates will be identified by transcriptional profiling of the two populations. Aim 3 will establish the functional impact of mast cell activation in each region wit...