Monocytes are circulating myeloid immune precursor cells that are generated in the bone marrow. Mature monocytes are released into the circulation and, in case of need, recruited to peripheral sites of inflammation to differentiate into monocyte-derived effector cells. In absence of overt inflammation, monocytes also extravasate into selected tissues, where they complement tissue-resident macrophage compartments. Adjustment of these homeostatic monocyte infiltrates to local environment is critical to maintain health, as best established for the intestine. Defined gene expression changes that differ between gut segments presumably help strike the fine balance between the crucial function of these monocyte-derived macrophages as tissue rheostats and their detrimental hyperactivation. Environmental factors that dictate local monocyte differentiation remain incompletely understood. Definition of the latter could aid our general understanding of in vivo monocyte functions and their relation to inflammatory disorders. In this review, we summarize recent advances in our understanding of monocyte subsets, their differentiation into tissue macrophages, and selected contributions of monocyte-derived cells to steady-state physiology. Moreover, we will discuss emerging evidence for an intriguing bifurcation of monocyte development in the bone marrow and potential functional implications. Emphasis will be given to points of controversies, but we will largely focus on the healthy organism. For a discussion of monocyte and macrophage contributions to inflammatory conditions, we refer the reader to other dedicated reviews.

单核细胞是在骨髓中产生的循环骨髓免疫前体细胞。成熟的单核细胞被释放到循环中，并在需要时被募集到炎症的外周部位以分化为单核细胞衍生的效应细胞。在没有明显炎症的情况下，单核细胞也会外渗到选定的组织中，在那里它们补充组织驻留的巨噬细胞区室。调整这些稳态单核细胞浸润到局部环境对于维持健康至关重要，这对于肠道来说是最好的。肠道区段之间不同的确定基因表达变化可能有助于在这些单核细胞衍生的巨噬细胞作为组织变阻器的关键功能与其有害的过度活化之间取得良好的平衡。决定局部单核细胞分化的环境因素仍未完全了解。后者的定义可以帮助我们大致了解体内单核细胞功能及其与炎症性疾病的关系。在这篇综述中，我们总结了我们在理解单核细胞亚群、它们分化为组织巨噬细胞以及单核细胞衍生细胞对稳态生理学的选择性贡献方面的最新进展。此外，我们将讨论有关骨髓中单核细胞发育的有趣分叉和潜在功能影响的新证据。重点将放在有争议的地方，但我们将主要关注健康的有机体。有关单核细胞和巨噬细胞对炎症的贡献的讨论，我们建议读者参考其他专门的评论。