Pathogenesis of malignant histiocytic lymphoma induced by silica in a colony of specificpathogen-free Wistar rats.

The occurrence of malignant histiocytic lymphomas (MHLT) after an intrapleural inoculation of silica and the presence of a hyperplastic or granulomatous reaction to the silica were confirmed in cesarean-derived barrier-maintained inbred W rats. A specific hyperplastic reaction in the silica granulomas preceded the appearance of MHLT. Hyperplastic changes were observed when the absolute monocyte count at weeks 25-30 was slightly higher than that seen in the controls. Silica injected intrapleurally penetrated the thymus tissue but the carbon and coal dusts and the silica, which was administered by other routes, did not involve the thymus. Also, silica was the sole dust, when introduced into the coelomic cavities that produced MHLT, which grew rapidly on entering the thymus. Rats that died with MHLT had widespread dissemination with malignant foci situated most consistently above the diaphragm.