Abstract
The brain is the most important and complex organ in most living creatures which serves as the center of the nervous system. The function of human brain includes controlling of the motion of the body and different organs and maintaining basic homeostasis. The disorders of the brain caused by a variety of reasons often severely impact the patients' normal life or lead to death in extreme cases. Monocyte is an important immune cell which is often recruited to the brain in a number of brain disorders. However, the role of monocytes may not be simply described as beneficial or detrimental. It significantly depends on the disease models and the stages of disease progression. In this review, we summarized the current knowledge about the role of monocytes and monocyte-derived macrophages during several common brain disorders. Major focuses include ischemic stroke, Alzheimer's disease, multiple sclerosis, intracerebral hemorrhage, and insomnia. The recruitment, differentiation, and function of monocyte in these diseases are reviewed.
Highlights
The human brain is the most sophisticated organ which orchestrates the behaviors of the body and internal organs
Monocyte can develop into microglia during Alzheimer’s disease (AD) progression, and it was shown that these microglial cells differentiated from BM-derived progenitor cells are more efficient in phagocytizing Aβ compared with resident microglia [53]
Monocytes play a role contributing to the inflammation which is detrimental for disease progression; on the other hand, monocyte can be protective due to involvement in the clearance of the hematoma and the functional recovery after intracerebral hemorrhage [83]
Summary
The human brain is the most sophisticated organ which orchestrates the behaviors of the body and internal organs. Tissue-resident macrophages reside in the brain under normal conditions, including microglia and perivascular macrophages. A special focus is given on the roles of recruited monocytes and subsequentially differentiated macrophages during major brain disorders. Monocytes are classified into two different subsets; one subset demonstrated the following pattern on flow cytometry CX3CR1lowCCR2hiGr1+ and is often regarded as inflammatory or classical monocyte (Ly6Chi population); the other monocyte subset CX3CR1hiCCR2-Gr1- (Ly6Clow population) is normally recruited to noninflamed tissues demonstrating a patrolling behavior and is alternatively named patrolling or nonclassical monocyte [2]. The recruit of nonclassical monocyte is reported to be CX3CR1 dependent [6, 7], and this signaling plays an important role during monocyte survival [8]. It should be noted that a substantial number of literatures exist on the roles of monocyte during brain disorders, and this review does not mean to be all inclusive
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