Comparative studies on the histopathogenesis of experimentally induced lesions of myocardial Aschoff body type in rabbits and of many myocardial Aschoff bodies from several active rheumatic fever patients have revealed the following:

Almost invariably these experimental lesions and very frequently the myocardial Aschoff bodies studied in their early stages have been shown to originate in and evolve from lesions of heart muscle fibers.

The mono-, multi-, and non-nuciested cell masses, most characteristic of myocardial disease of the rheumatic type, appear to be damaged muscle fibers their fragments, and syncytial cell masses of probable muscular origin that proliferate from beneath the sarcolemma and in the tracks of damaged muscle fibers in reaction to that damage.

In addition to destructive changes in cardiac muscle fibers an attempt at myofiber regeneration may occur in some myocardial Aschoff bodies.

The most distinguishing histologic feature of the myocardial Aschoff bodies in rheumatic heart disease are the peculiar lesions of muscle fibers. Therefore, it is proposed that they be designated as myofiber Aschoff bodies in order to indicate their origin more accurately.

The results of these investigations contrast with the widely accepted theory that all myocardial Aschoff bodies originate as injured interstitial collagen, and that, as they evolve, they consist of damaged interstitial collagen intermingled with cells of non-myogenic derivation that proliferated in response to that collagen injury.

These studies, furthermore, provide evidence that experimental homologues of rheumatic myocardial Aschoff bodies have been induced in a very few among many rabbits subjected to reposted focal infections with group A streptococci of different serological types. Hence, they support the concept that the myocardial Aschoff bodies of rheumatic fever are induced by repeated infections with group A streptococci of several different serological types; even though only a certain few among the many patients so infected develop these lesions.

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