REGENERATION OF SKELETAL-MUSCLE IN 2 TELEOST FISH - SPARUS-AURATA ANDBRACHYDANIO-RERIO

Regeneration of skeletal muscle was studied in the sea bream Sparus au rata, in which extensive postlarval muscle hyperplasia contributes to its large adult size, and in the zebrafish Brachydanio rerio, which sh ows little post-larval hyperplasia and reaches only a small adult size . Small mechanical lesions of body wall muscle were made under general anaesthesia, and the progress of subsequent regeneration was assessed at various intervals by histology and electron microscopy (for genera l morphology), by immunostaining for desmin and myosin isoforms (to id entify the phenotype of new fibres), and by 5'-bromo-2'-deoxyuridine ( BrdU) incorporation (to identify proliferating cells). Despite the dif ference in normal growth-related hyperplasia in these fish, a vigorous regeneration occurred in both species, giving rise to new fibres with an initial myosin composition that differed from that in mature fast- white fibres. However, species differences in myosin expression in the se fibres suggest that they may have derived from different myoblast p opulations. In sea bream, myosin expression in regenerating fibres res embled that seen in new fibres produced in post-larval white muscle, w hereas in the zebrafish it resembled that of the primitive monolayer f ibres formed during embryonic development. Subsequently, most regenera ting fibres gradually transformed into the mature fast-white phenotype in both species.