Ultrastructure of benign chondroblastoma.

Electron microscopic examination of three chondroblastomas showed two main cell types, namely pale and dark cells. Multilobulate nuclei characteristic of typical neoplastic chondroblasts were found in both the cell types. Besides differences in cytoplasmic and nuclear density, the cells also differed in nuclear shape and in proportions of various cytoplasmic organelles. Small pale cells resembled prechondroblasts of embryonic cartilage. Large pale cells showed features of chondroblasts. From pale cells of the above type, smooth transitions were observed to the dark cells, possessing dense, less regularly shaped nuclei and typically a larger amount of rough endoplasmic reticulum. Some of the dark cells may represent cells undergoing regressive alterations, but the majority of them seem to represent chondroblasts with imperfect maturation towards chondrocytes. Such findings suggest that benign chondroblastoma is composed of cartilaginous cells with various degrees of differentiation. The finding of occasional cilia is reported.