Ultrastructure of naturally occurring subcutaneous nodule in acute rheumatic fever.

Electron microscopic evaluation of subcutaneous nodules excised from two cases of acute rheumatic fever showed an electron dense and smudgy material which had a distinct filamentous appearance in places. The latter was recognized as collagen. This corresponded to the fibrinoid necrosis seen under the light microscope. In addition, several histiomonocytic cells and fibroblasts were seen. A large number of cells were degenerated. A fair degree of resemblance of the nodule both by light and electron microscopy to the rheumatoid nodule was noted.