Immunocytochemical analysis of the perinatal development of cat masseter muscle using anti-myosin antibodies.

The developmental changes in myosin gene expression in the masseter muscle of embryonic and juvenile kittens were examined immunocytochemically using anti-myosin heavy chain antibodies of various specificities. In the mature cat, this muscle contains only two phenotypes, the majority of fibres are superfast, the rest being slow fibres. In foetal tissues, the histological appearance of bundles of myotubes, comprising a large central myotube surrounded by a rosette of smaller myotubes, strongly suggest the existence in the jaw muscle of primary and secondary fibres during development. Immunocytochemical data are consistent with the hypothesis that there are four types of fibre; two types of primary fibre as well as two types of secondary fibre. (1) Slow primaries stain strongly with an anti-slow myosin antibody throughout the period under study. These fibres transiently express embryonic but not foetal myosin. (2) Superfast primaries stain for embryonic/foetal and slow myosins in the perinatal period but progressively replace these myosins with superfast myosin during postnatal development. (3) Superfast secondaries initially express embryonic/foetal myosins, but later, beginning around the time of birth progressively replace these myosins with superfast myosin. These fibres do not express slow myosin. (4) Slow secondaries, which initially also express embryonic/foetal myosins, but which postnatally express slow or slow and superfast myosins and express only slow myosin in the adult. These four types of fibres are homologous to the four isotypes of limb muscle fibres and may be derived from distinct lineages of myoblasts.