Our understanding on the function of microglia has been revolutionized in the recent 20 years. However, the process of maintaining microglia homeostasis has not been fully understood. In this study, we dissected the features of spinal microglia repopulation following an acute partial depletion. By injecting intrathecally Mac-1-saporin, a microglia selective immunotoxin, we ablated 50% microglia in the spinal cord of naive mice. Spinal microglia repopulated rapidly and local homeostasis was re-established within 14 days post-depletion. Mac-1-saporin treatment resulted in microglia cell proliferation and circulating monocyte infiltration. The latter is indeed part of an acute, transient inflammatory reaction that follows cell depletion, and was characterized by an increase in the expression of inflammatory molecules and by the breakdown of the blood spinal cord barrier. During this period, microglia formed cell clusters and exhibited a M1-like phenotype. MCP-1/CCR2 signaling was essential in promoting this depletion associated spinal inflammatory reaction. Interestingly, ruling out MCP-1-mediated secondary inflammation, including blocking recruitment of monocyte-derived microglia, did not affect depletion-triggered microglia repopulation. Our results also demonstrated that newly generated microglia kept their responsiveness to peripheral nerve injury and their contribution to injury-associated neuropathic pain was not significantly altered.Although neurons in the central nervous system (CNS) have limited capacity for regeneration, glial cells exhibit remarkable self-renewal potential. Aroused from yolk sac progenitors that populate the CNS during embryogenesis, microglia in adulthood has been well recognized for their capability in preserving local homeostasis. Failure to keep up microglia in their normal physiological states leads to alteration in the stability of CNS micro-environment, as microglia are not only overseers of pathological disturbances 1,2 they also have physiological roles in normal CNS function 3,4 . However, the question of how microglia strive to maintain the integrity of the cell population is intriguing and unresolved, it has drawn much attention in recent research of microglia cell biology. Several research groups have investigated microglia repopulation after depletion in the brain parenchyma using genetic and/or pharmacological approaches. The main findings have identified the CNS resident microglia as the cell population responsible for re-establishing the CNS microglia compartment. Elmore et al. 5 reported that following depletion by blocking colony-stimulating factor1 receptor (CSF1R) signaling, microglia can repopulate solely through proliferation of nestin-positive, resident cells which then differentiate into microglia. The notion that microglia repopulation relies fully on CNS resident cells is further supported by the group of Bruttger 6 where Cx3cr1CreER :iDTR system has been used to ablate microglia cells. The participation of bone marrow-derived cells in the regeneration process...

Temporomandibular disorder (TMD) is a set of heterogeneous musculoskeletal conditions involving the temporomandibular joint (TMJ) and/or the masticatory muscles. Up to 33% of the population has had at least 1 symptom of TMD with 5% to 10% of them requiring treatment. Common symptoms include limited jaw movement, joint sound, and pain in the orofacial area. Once TMD becomes chronic, it can be debilitating with comorbidities that greatly reduce one's overall quality of life. However, the underlying mechanism of TMD is unclear because of the multicausative nature of the disease. Here, we report a novel mouse model of TMD where a bite block was placed in between the upper and lower incisors such that the mouth was kept maximally open for 1.5 hours per day for 5 days. After sustained mouth opening, mice developed persistent orofacial mechanical allodynia and TMJ dysfunction. At the cellular level, we found masseter muscle dystrophy, and increased proteoglycan deposition and hypertrophic chondrocytes in the mandibular condyle. Increased F4/80 macrophages were also observed in the masseter muscles and the TMJ posterior synovium. We also found ATF3 neuronal injury and increased F4/80 macrophages in the trigeminal ganglia. Microglia activation was observed in the trigeminal subnucleus caudalis. Inhibiting macrophage and microglia activation with a colony stimulating factor-1 receptor inhibitor prevented the development of orofacial mechanical allodynia, but not TMJ dysfunction. This study suggests that mouth opening for an extended period during dental treatments or oral intubations may risk the development of chronic TMD and inflammation associated with macrophage and microglia in the tissue and trigeminal system contributes to the development of TMD pain.