A murine distal femoral epiphysis ischemia model reveals spatiotemporal stratification of necrotic bone marrow clearance and associated inflammatory responses.

BACKGROUND

Efficient clearance of necrotic marrow is essential for repair after epiphyseal ischemia, yet the tissue-level spatiotemporal evolution of this process and its immunoregulatory mechanisms remain incompletely defined.

METHODS

Using a refined juvenile ischemic osteonecrosis (JIO) model of the murine distal femoral epiphysis-with medial plus lateral approaches guided by mapped vascular entry points-we fully interrupted epiphyseal perfusion and harvested tissue at postoperative days 1-28. Coronal sections along the vascular entry axis underwent H&E/TUNEL and zonal IHC/IF (Ly6G, myeloperoxidase (MPO), F4/80, EMCN, iNOS, CD206). An in-vitro assay tested whether recombinant MPO (1 μg/mL) modulates LPS + IFN-γ-induced M1 polarization in RAW264.7 cells.

RESULTS

We consistently observed a necrosis-repair interface characterized by a centripetal tri-zonal progression: fibrotic zone → resorption zone → necrotic core. Apoptotic marrow cells-including resident neutrophils-transitioned into secondary necrosis within the resorption zone. This zone demonstrated enrichment of EMCN+ microvessels alongside a reduction in hematoxylin-positive nuclear material, indicative of chromatin disposal supported by revascularization processes. At the fibrotic margin, F4/80+ macrophages engulfed MPO+, and iNOS+ (M1) macrophages outnumbered CD206+ (M2). Furthermore, recombinant MPO enhanced M1 polarization of RAW264.7 cells in vitro.

CONCLUSIONS

The JIO necrosis-repair interface adheres to a distinct spatiotemporal program in which apoptotic cells transition into secondary necrosis concurrently with revascularization efforts. Cells undergoing secondary necrosis are subsequently eliminated during the fibrovascular replacement processes. Notably, neutrophil-derived MPO spatially associates with M1-skewed macrophages in vivo and enhances M1 polarization in vitro; this suggests an immunoregulatory axis exists between neutrophils and macrophages at the interface. Strategies aimed at enhancing the clearance of secondary necrosis neutrophils and modulating this axis may facilitate inflammatory resolution and improve epiphyseal repair.