Ultrastructural Observations of Skin Lesions in MRL Mice —Dermal Infiltrations and Capillary Changes

The skin lesions of MRL/l mice were examined by electron microscopy and the following changes were disclosed in the dermis. Fibroblasts and mast cells were increased in number and activated. A major part of the infiltrating mononuclear cells was morphologically T‐lymphocytes. The capillaries, degenerated to varying degrees with indistinct basal laminae, were remarkably increased, and exudative amorphous material was deposited around them. Neither electron‐optically dense deposits around the vessels suggesting immune deposits nor tubulo‐reticular structures in the endothelial cells were detected. The basic lesions occurring in the dermis of MRL/l mice seem to be primary injuries of the capillary endothelial cells and consequential exudative changes of perivascular areas, similar to those of erythematous lesions in human systemic lupus erythematosus.