Canine muscle fiber types and susceptibility of masticatory muscles to myositis.

The myofiber type composition was studied in 42 different muscles of the dog to determine if there are unique features that might explain the preferential involvement of the muscles of mastication by inflammatory myopathies. The principal myofiber types for most muscles studied were type 1 and type 2A and, to a lesser extent, type 2C, whereas the dorsal group of muscles innervated by the mandibular nerve (Mm. temporalis, and tensor veli palatini) was composed only of type 2C myofibers and a variant of the type 1 myofiber whose staining intensity was not fully reversed after preincubation in acid media. The distribution of this myofiber type composition was associated with the innervation and embryologic development of the dorsal muscles innervated by the mandibular nerve. This unique myofiber type composition could provide the basis for the preferential susceptibility of these muscles to agents (e.g., immune and/or infectious) that produce myositis; however, further studies are required to assess that possibility.