Origin of Aschoff nodule. An ultrastructural, light microscopic and histochemical evaluation

Excised left atrial appendages were subjected to light and electron microscopy and histochemical analysis. Aschoff nodules (AN) showed strong acid phosphatase and nonspecific esterase activity. Ultrastructurally, the Aschoff cells had several features of fibroblasts and epithelioid cells. Rough endoplasmic reticulum (RER) was striking in most cells. Dilated cisterns of RER containing homogeneous material were also frequent. Mitochondria and ribosomes were present. AN thus seem to arise from mesenchymal tissues of the heart. It is likely that cross-reactivity between streptococcal antigen and myocardial connective tissue causes the cardiac histocyte/macrophage to transform to cells having features of epithelioid cells and/or fibroblasts.