Histoid leprosy. An ultrastructural observation.

A young active histoid nodule from an untreated patient with lepromatous leprosy was examined by electron microscopy. The only area where spindle-shaped cells predominated was near the pseudocapsule, and these cells were the only ones that were associated with the presence of collagen. The main mass of the granuloma was composed of young macrophages which showed evidence of a higher cell turnover rate than in ordinary lepromatous leprosy. Among the macrophages were plasmacytoid cells. Histoid lesions are thought to be essentially hyperactive, but the nature of the stimulus is unknown.