Monocyte cluster identification during the early stage of fracture healing in an ovariectomized mouse model

Monocytes play a pivotal role in the process of fracture healing, particularly during the early phase. However, the specific functions of their cellular subsets and the alterations that occur in the context of osteoporotic fracture (OPF) remain poorly elucidated.

An OPF model was established using ovariectomized (OVX) C57BL/6 J mice, with sham-operated mice serving as controls. Subsequent single-cell RNA sequencing (scRNA-seq) analysis was performed to identify and characterize monocyte populations within the early fracture microenvironment.

scRNA-seq analysis revealed that monocytes could be classified into four distinct subsets. Monocytes-1 and monocytes-2 were identified as Ly6C⁺ monocytes; specifically, monocytes-1 differentiated into M1 macrophages to mediate inflammatory responses, whereas monocytes-2 proliferated and served as a cellular reservoir to support the differentiation of monocytes-1. Monocytes-3 and monocytes-4 exhibited the potential to differentiate into M2 macrophages. Functionally, monocytes-3 primarily constrained the magnitude of inflammatory responses, while monocytes-4 were involved in orchestrating tissue repair processes, such as angiogenesis. In the local bone tissues of OVX mice at the early fracture stage, all monocyte subsets exhibited a tendency toward reduced abundance.

This study reveals a high degree of functional heterogeneity among monocyte subsets during early fracture healing. The reduction in monocyte subsets in OVX mice impairs the critical pathological transition from inflammatory responses to tissue repair. This impairment may constitute a partial mechanistic basis for the delayed and compromised healing observed in OPF.