DIFFERENTIATION AND GROWTH OF MUSCLE IN THE FISH SPARUS-AURATA (L) .1. MYOSIN EXPRESSION AND ORGANIZATION OF FIBER TYPES IN LATERAL MUSCLE FROM HATCHING TO ADULT

Post-hatching development of lateral muscle in a teleost fish, Sparus aurata (L) was examined. At hatching only two fibre types were present ; several layers of mitochondria-poor, myofibril-rich deep muscle fibr es surrounded the notochord and were covered by a superficial monolaye r of mitochondria-rich, myofibril-poor fibres. A third ultrastructural ly distinct fibre type first appeared as one or two fibres located jus t under the lateral line at 6 days post-hatching. This type, which gra dually increased in number during larval life, contained a slow isofor m of myosin, identified by mATPase staining and immunostaining with my osin isoform-specific antibodies. Deep muscle fibres - the presumptive fast-white type-contained a fast myosin, and superficial monolayer fi bres an isoform similar but not identical to that in adult pink muscle fibres. The only fibres present during larval life which showed a cle ar change in myosin expression were the superficial monolayer fibres, which gradually transformed into the slow type post-larvally. Pink mus cle fibres first appeared near the end of larval life. Both slow and p ink muscle fibres remained concentrated around the horizontal septum u nder the lateral line during larval life, expanding outwards towards t he apices of the myotomes only after metamorphosis. Between 60 and 90 days very small diameter fibres with a distinct mATPase profile appear ed scattered throughout the deep, fast-white muscle layer, giving it a 'mosaic' appearance, which persisted into adult life. A marked expans ion in the slow muscle layer began at the same time, partly by transfo rmation of superficial monolayer fibres, but mainly by addition of new fibres both on the deep surface of the superficial monolayer and clos e to the lateral line. The order of appearance of these fibre types, t heir myosin composition, and the significance of the superficial monol ayer layer are discussed and compared to muscle fibre type development in higher vertebrates.