[Histological characteristics of head and neck paragangliomas]

Paragangliomas arise from the extra-adrenal paraganglion system. Histologically, paragangliomas are usually easy to diagnose, with well-defined characteristics. These lesions are clearly delimited and highly vascular and are composed of cell balls (Zellballen) separated by thin fibrous septa. These cell balls are composed of two types of cells: chief cells and sustentacular cells. Other, less frequent patterns, which are nearly always focal, can also be found and hamper diagnosis: angiomatoid, fusocellular and clear cell. Some paragangliomas show intense fibrosis, which can compress and distort the cell balls, giving rise to a pseudoinfiltrative appearance (sclerosing paragangliomas). With immunohistochemical techniques, the chief cells are positive for neuroendocrine markers (neuron specific enolase, chromogranin A, synaptophysin, serotonin) while sustentacular cells are positive for S-100 protein. Ultrastructurally, the chief cells contain neurosecretory granules with dense centers and simple intercellular junctions without desmosomes. From a practical point of view, paragangliomas can be divided into three groups: non-invasive (circumscribed or encapsulated), locally invasive and metastatic. Although some invasive tumors can be fatal, there are no histological data that can predict the malignancy of paragangliomas, and the only absolute criterion for malignancy is the presence of metastasis.