Crystalloids in angiomyolipoma: 1. A previously unnoticed phenomenon of renal angiomyolipoma occurring at a high frequency

We present a description of unique crystalloids in renal angiomyolipoma that have not previously been reported. The crystalloids cannot be identified by hematoxylin-and-eosin staining. Detailed observation after diastase treatment followed by PAS staining revealed needle- and rod-like crystalloids, which were clearly seen even by light microscopy, in 11 of 17 patients. Their appearance was characterized by the following phenomena: (a) They appeared mainly in large epithelioid smooth-muscle cells; (b) they appeared at a relatively high frequency at sites where smooth-muscle cells showed diffuse proliferation and where a hemangiopericytic pattern was observed; (c) they were often detected easily even at a site with a sarcomatous appearance; and (d) PAS-positive, diastase-resistant granules were often observed by light microscopy in the vicinity of crystalloids in all 17 patients. Electron-microscopic observation of one patient also revealed characteristic crystalloids. Prior to our study, only one patient had been reported to show crystalloids by electron microscopy, and the crystalloids were interpreted as renin. However, our study used Bowie's staining and immunohistochemistry to prove they were not renin. The nature of the crystalloids still needs to be elucidated. The fact that they closely resemble structures seen in alveolar soft part sarcoma provides one clue to their identification.