Venous vasculature drives neovascularization and stroma formation in pancreatic neuroendocrine tumors via intussusceptive angiogenesis and vein intravasation.

The microenvironment of pancreatic neuroendocrine tumors (PanNETs) has been extensively studied; however, research on their venous vasculature has largely focused on tumor invasion and metastasis. This study aims to (a) evaluate the role of veins/venules in PanNET neovascularization, tumor intravasation, and stromal tract (trabecula) formation, including the potential involvement of intussusceptive angiogenesis (IA); and (b) compare the trabeculae observed in PanNETs with those found in hepatic cavernous hemangiomas (HCHs) where IA in veins plays an important role. To achieve these objectives, we conducted an integrated morphological approach encompassing primary PanNETs (n=42), hepatic metastases of PanNETs (n=4), and HCHs (n=11). Our findings in both primary and metastatic PanNETs reveal (a) the involvement of veins/venules in tumor neovascularization, with IA acting synergistically with sprouting angiogenesis, through the formation of pillars, meshes, and complex meshes (vessels that encapsulate tumor clusters-endothelium-coated tumor clusters); (b) the development of connective tissue around the neo-vasculature, potentially involving adventitial CD34-positive stromal cells/telocytes; and (c) a notable architectural resemblance between the trabeculae of PanNETs and those of HCHs. In conclusion, this work highlights the pivotal role of the preexisting venous vasculature in PanNET neovascularization, tumor intravasation, and stroma formation, with active participation of IA. These findings provide a pathophysiological foundation for future in-depth molecular investigations and may pave the way for new studies on therapeutic strategies targeting angiogenic mechanisms.