Microglia constitute the largest population of parenchymal macrophages in the brain and are considered a unique subset of central nervous system glial cells owing to their extra-embryonic origins in the yolk sac. During development, microglial progenitors readily proliferate and eventually colonize the entire brain. In this Review, we highlight the origins of microglial progenitors and their entry routes into the brain and discuss the various molecular and non-molecular determinants of their fate, which may inform their specific functions. Specifically, we explore recently identified mechanisms that regulate microglial colonization of the brain, including the availability of space, and describe how the expansion of highly proliferative microglial progenitors facilitates the occupation of the microglial niche. Finally, we shed light on the factors involved in establishing microglial identity in the brain.

小胶质细胞构成大脑中最大的实质巨噬细胞群，由于其胚胎外起源于卵黄囊，因此被认为是中枢神经系统胶质细胞的独特子集。在发育过程中，小胶质细胞祖细胞很容易增殖并最终定居在整个大脑中。在这篇综述中，我们强调了小胶质细胞祖细胞的起源及其进入大脑的途径，并讨论了它们命运的各种分子和非分子决定因素，这可能会告知它们的特定功能。具体来说，我们探索了最近发现的调节大脑小胶质细胞定植的机制，包括空间的可用性，并描述了高度增殖的小胶质细胞祖细胞的扩张如何促进小胶质细胞生态位的占据。最后，我们阐明了在大脑中建立小胶质细胞身份所涉及的因素。