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Review
. 2023 Aug 5;12(8):1091.
doi: 10.3390/biology12081091.

Towards an Understanding of Microglia and Border-Associated Macrophages

Takumi Taketomi  1 , Fuminori Tsuruta  1   2   3   4
Affiliations
Review

Towards an Understanding of Microglia and Border-Associated Macrophages

Takumi Taketomi et al. Biology (Basel). .

Abstract

The central nervous system (CNS) plays a crucial role in regulating bodily functions by sensing and integrating environmental cues and maintaining proper physiological conditions. Recent research has revealed that CNS functions are closely coordinated with the immune system. As even minor disturbances of the immune system in the CNS can lead to various dysfunctions, diseases, or even death, it is highly specialized and segregated from that in peripheral regions. Microglia in the parenchyma and macrophages at the interface between the CNS and peripheral regions are essential immune cells in the CNS that monitor environmental changes. Recent omics analyses have revealed that these cells exhibit highly heterogeneous populations. In this review, we summarize the functions and diversity of microglia in the brain parenchyma and those of macrophages in the border regions, such as the meninges, perivascular spaces, and choroid plexus.

Keywords: aging; border-associated macrophages; brain parenchyma; choroid plexus; development; homeostasis; meninges; microglia; neurodegeneration; neuroinflammation; neurovascular space.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Changes in the state of microglia and macrophages in the brain. During embryogenesis, cells derived from the yolk sac differentiate into microglia in parenchyma and border-associated macrophages (BAMs) in CNS border regions, such as the meninges, perivascular regions, and choroid plexus. The functions and characteristics of microglia and BAMs are diverse and contextually dependent. It has been reported that microglia undergo functional changes concomitant with morphological changes. However, there is insufficient information regarding the ontogeny of BAMs. PCs: microglial and BAM precursor cells (MG/BAM PCs).
Figure 2
Figure 2
BAMs population in CNS border regions. CNS border regions are highly specialized anatomical structures. BAMs have heterogeneous populations to adapt to each CNS border region, such as the perivascular regions, choroid plexus, and meninges (dura mater, arachnoid mater, pia mater). The dura mater and choroid plexus have fenestrated vessels, allowing replacement by bone marrow–derived macrophages. Similarly, SLYM is connected to the venous sinus wall, allowing infiltration of bone marrow–derived macrophages. Skull channels are the direct vascular channels that connect the skull bone marrow and dura mater, allowing immune cell migration. Ventricle, subarachnoid space, and perivascular regions are filled with cerebrospinal fluid (CSF), dMΦ, dural macrophage, SLYM, subarachnoid lymphatic-like membrane, sdMΦ, subdural macrophage, pvMΦ, perivascular macrophage, cpstMΦ, choroid plexus stromal macrophage, and cpepMΦ, choroid plexus epiplexus macrophage.
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