Ultrastructure of the spleen in malignant histiocytosis.

We examined the ultrastructure of the spleen surgically removed from a patient with the in vivo diagnosis of malignant histiocytosis. The findings showed sinuses greatly dilated and lined by large atypical histiocytic cells. Erythrophagocytosis was prominent. These findings, as well as the histologic features of the bone marrow and later autopsy specimens, confirmed the diagnosis of malignant histiocytosis. The somewhat unusual confinement of histiocytes to sinusoids raised the possibility that this may represent a rare subvariant, histiocytic sinus reticulosis, first described by Robb-Smith in his original paper.