(A, B) Electron micrographs illustrate numerous blood vessels and vessel-like structures (black arrows), cells within the FVM stroma that are not associated with vascular structures (white arrows), as well as large areas of fibrosis (Fi) within a FVM resected from clinical case 1. Ultrastructural analysis reveals vessels with varying degrees of vascular abnormalities (C and D), with multiple intercellular junctions between endothelial cells (black arrows), irregular perivascular cell investment (P), swollen mitochondria (*), and abnormal plasma membrane infoldings (white arrowheads). n=nucleus, Lu=Lumen. Scale bars: (A, B)=10 µm; (C, D)=2 µm. To access the data, click or select the words “Appendix 2.”