Characterization of eosinophilic hyaline droplets in schwannoma

In schwannoma, degenerative structures, such as bizarre hyperchromatic nuclei, lipofuscin deposition, hemorrhage, edema, etc., are frequently encountered and are regarded as hallmarks of the benign nature of the neoplasm. Hyaline globules or eosinophilic hyaline droplets (EHDs) have been observed in a large variety of tumors, but they are rare histological manifestations of schwannoma, and the details have not been described as yet. We reviewed 171 cases of schwannoma of conventional histology. We classified them based on the sites where they occurred as follows: 43 acoustic nerves, 9 other cranial nerves, 30 spinal nerve roots, and 89 soft parts. We found EHDs in 8/43, 1/9, 1/30, 2/89 cases, respectively. The droplets were located in both the perikarya and processes. Ultrastructurally, they were electron-dense, round to ovoid organelles, and indistinguishable from secondary lysosomes. MIB-1 immunolabeling of the droplet-bearing cells was almost negative; however, they maintained a viable nuclear appearance without karyorrhexis, pyknosis or apoptotic changes. We therefore conclude that EHD is a new member of degenerative structures in schwannoma, and a hallmark of low growth potential. Furthermore, their higher frequency in acoustic schwannoma may suggest their distinct nature from those originated in the other sites. Possible relevance of EHDs and cellular senescence is also discussed.