The ultrastructure of topical cutaneous calcinosis.

Mineralized plaques, which develop at the site of repeated subcutaneous injections of 100 μg KMnO4/0.2 H2O in rats, were investigated by electron microscopy. The newly formed, delineated, white plaque tissue at the injection site consisted of numerous, mostly unaltered fibroblasts and collagen fibers, without participation of inflammatory cells. Some signs of cell injury were found in the center of the lesions. Numerous, irregularly distributed, small, mineralized foci were seen near the fibroblasts. These were formed by aggregation of small needle-like units (50 A in diameter and 0.05–2.0 μm long). These needle-shaped units were found either in vesicular, cell derived structures, considered to be shed cell processes or cell fragments, or on collagen fibers. Intramitochondrial deposits of such needle-like units were seen frequently. Fusion of smaller mineralized foci to larger plaques occured and then needle-shaped units were seen at the periphery of the electron-dense lesions. Hypotheses concerning the mechanism of experimental cutaneous calcinosis (soft tissue mineralization) are discussed and related to the findings of this study. Probable intracellular crystal deposition and mineralization in cell-derived structures were shown for the first time in topical cutaneous calcinosis.