Ultrastructural features of tumor‐associated mast cells in parasympathetic paragangliomas (chemodectomas) of the neck

The mechanisms of the pathogenesis of neck paraganglioma (PGL) and the possible role of mast cells (MCs) in its development and metastasis are still poorly understood. We analyzed MCs' morphologic characterization, activation, and the properties of their cytoplasmic/released granules in PGLs, using light and transmission electron microscopy. Paragangliomas showed a large tumor‐associated MC population both in the connective tissue layers of the tumor and between the tumor cells. Notably, MCs were presented by a high expression of specific proteases, size variation, polymorphism, and variable ultrastructural phenotype of granules. A massive number of granules were released surrounding the degranulated MCs while the integrity of MC membrane was maintained. Granules were electron‐dense with or without a membrane, ranging from 0.2 to 0.8 μm in diameter. MC plasmalemma was not found at the site of MC‐collagen fibrils contact, whereas the secretome and fibrils were directly contacted. We observed direct and mediator‐based interactions between MCs and paraganglioma cells. The latter preserved their membrane integrity when MC granules were not in proximity. The effects of the MC secretome on the paraganglioma microenvironment demonstrated its pathogenetic role in tumor progression and allow its application to new diagnostic criteria and the development of protocols for personalized therapy. Research Highlights Ultrastructural analysis reveals novel regulatory effects of mast cells via diverse secretory pathways on the pathogenesis of parasympathetic paraganglioma, including fibrous extracellular matrix remodeling and mediator‐based interactions between MCs and cells of the tumor microenvironment. MCs are found with variable morphology, size of granules, and ultrastructural phenotypes in paraganglioma. They are abundant between the tumor cells and in the environment and can be found degranulated.