An immunohistochemical study of the middle ear muscles of some carnivores and primates, with special reference to the IIM and slow-tonic fibre types.

The middle ear muscles of several species of carnivores (cat, dog, fox, ferret and stone-marten) and some New World monkeys (Callithrix, Saimiri) and Old World monkeys (Cercopithecus, Macaca) were examined. The fibre type compositions of these muscles were determined by a combination of the standard histochemical myofibrillar ATPase method, and immunohistochemical techniques using myosintype-specific antisera. Immunohistochemically slow-tonic fibres were found in the stapedius muscles of only two carnivores, the ferret and stone-marten. In all the carnivores and the New World monkeys, tensor tympani muscle contained IIM, slow-tonic and slow-twitch fibres, but in the Old World monkeys it resembled stapedius muscle, and contained only Type I (slow-twitch) and IIA fibres. Thus, because all the species examined had IIM fibres in the jaw-closer muscles, this means that the common embryological origin of tensor tympani muscle and the jaw-closers does not necessarily result in tensor tympani muscle containing this fibre type even though IIM fibres occur only in first branchial arch muscles. This fact, together with other species differences in the fibre type composition of these muscles, shows that there is no typical composition of middle ear muscles in general, and suggests that the differences are related to very different functional requirements.