Design: A total of 12 samples were collected from focal and adjacent tissues with EMs and normal endometrial tissues. After tissue cells were dissociated, single-cell sequencing was performed by Seurat analysis. Results were further verified by immunohistochemistry, multiple immunofluorescence, and in vitro and in vivo experiments.

Setting: The consent form was signed before the operation and specimens were obtained during the operation.

Patients or Participants: Four patients with EMs and four patients with normal endometrium were included in the study.

Interventions: No intervention.

Measurements and Main Results: The results have identified 23 distinct cell populations, which were classified into 10 cell types according to marker gene expression. Among them, after cell subtype analysis of the highest proportion of fibroblasts (36%), 5 specific subtypes of fibroblasts were defined, and an inflammatory-associated fibroblast (FABP4⁺ FIB) was found in the lesion. Our results suggest that FABP4 was co-expressed with macrophage to fibroblast transition (MFT) cells and was correlated with the severity of fibrosis. TGF-β1 treated with Smad3 could upregulate the expression of FABP4 and COL1A1(a fibroblast marker) of BMDMs, leading to promote MFT, whereas knocking out Smad3 and treatment with highly selective TGF-β1 inhibitor could inhibit MFT. Similarly, TGF-β1 has been validated in animal experiments to increase the number of FABP4⁺ MFT cells and focal fibrosis in endometriotic lesions.

Conclusion: This study has revealed a novel mechanism of macrophage fibroblast transformation in fibrosis in EMs, which provides new ideas for further exploration of the fibrotic mechanism and the treatment in EMs.