Ultrastructure of cells of the mononuclear-phagocyte series (MPS) across the leprosy spectrum.

A systematic ultrastructural study of cells of the MPS across the spectrum of leprosy has been carried out. Graded changes in macrophage ultrastructure from the lepromatous to the tuberculoid poles have been shown. Mycobacteria-filled macrophages in lepromatous leprosy are characterised by long cell processes, whereas in borderline tuberculoid leprosy these cells have a rounded appearance and are mainly characterised by numerous intracellular vacuoles. In borderline leprosy, macrophages have an intermediate appearance. Cells of the MPS containing abundant endoplasmic reticulum were only seen in typical "epithelioid cell" tuberculoid granulomas in "BT in reaction" and in the Mitsuda reaction. Epithelioid cell granulomas in other forms of BT leprosy contained activated macrophages.