DIFFERENTIATION AND GROWTH OF MUSCLE IN THE FISH SPARUS-AURATA (L) .2. HYPERPLASTIC AND HYPERTROPHIC GROWTH OF LATERAL MUSCLE FROM HATCHINGTO ADULT

Post-hatching growth of lateral muscle in a teleost fish, Sparus aurat a (L) was studied morphometrically to identify and quantify muscle fib re hyperplasia and hypertrophy, and by in vivo nuclear labelling with 5-bromo-deoxyuridine to identify areas of myoblast proliferation. Musc le fibre types were identified principally by myosin ATPase histochemi stry and immunostaining, and labelled nuclei were identified at light and electronmicroscope level by immunostaining with a specific monoclo nal antibody. Hyperplastic growth was slow at hatching, but then incre ased to a maximum at the mid-point of larval life. Larval hyperplastic growth occured by apposition of new fibres along proliferation zones, principally just under the lateral line and in the apical regions of the myotome, but also just under the superficial monolayer at intermed iate positions. The first of these zones gave rise to slow and pink mu scle fibres, in a process which continued through into postlarval life . The other zones added new fibres to the fast-white muscle layer in a process which was exhausted by the end of larval life. Post-larvally, between 60 and 90 days posthatching, a new hyperplastic process start ed in the fast-white muscle as nuclei proliferated and new muscle fibr es were formed throughout the whole layer. This process resulted in a several-fold increase in the number of fast-white fibres over a few we eks, and then waned to very low levels in juveniles. Hyperplasia by ap position continued for some time postlarvally on the deep surface of t he superficial monolayer, but at this stage gave rise to slow fibres o nly. Hypertrophic growth occurred at all ages, but was the dominant me chanism of muscle growth only in the juvenile and adult stages. Mechan isms giving rise to these different growth processes in fish muscle ar e discussed, and compared with muscle development in higher vertebrate s.