Immunocytochemical markers (endothelial and histiocytic) and ultrastructure of primary aneurysmal bone cysts.

Since the first description of an aneurysmal bone cyst in 1942, the pathogenesis of this tumor-like lesion has been controversial. Aspects of interest for elucidating the nature of this lesion include the cellular linings of the aneurysmal cavity systems as well as the character and cellular composition of the stromal septa. To clarify these features, seven aneurysmal bone cysts were studied electron microscopically and immunocytochemically with endothelial (Factor VIII-related antigen, monoclonal endothelial marker) and histiocytic (alpha 1-antitrypsin, alpha 1-antichymotrypsin, lysozyme, acid phosphatase) markers. Both immunocytochemical and electron microscopic examination revealed that the aneurysmal cavernous spaces have no endothelial lining but are delimited by fibroblasts and histiocytic cell forms at varying stages of differentiation. These cell forms are also the main component of the mononuclear stroma cells of the septa between the aneurysmal cavities. From these findings it is concluded that the aneurysmal cavities are not vascular. The sinusoidal septal capillaries might play a special role in their pathogenesis. Due to the lack of basal membrane structures, rupture of the wall gives rise to erythrocyte extravasates, which can undergo secondary transformation into cysts.