Developmental transitions of myosin isoforms and organisation of the lateral muscle in the teleostDicentrarchus labrax (L.)

InDicentrarchus labrax (the sea bass) the differentiation of lateral muscle fibres occurs at different stages and in different ways in the superficial (red), intermediate (pink) and deep (white) regions of the myotome. At hatching the myotomes are composed of presumptive white and red fibres, the latter forming a superficial monolayer present only near the transverse septum. At this stage, differences between the fibre types are mainly ultrastructural. From their different reactions with isoform-specific antibodies to mullet myosin, and the appearance of histochemical mATPase activity, it appears that in both red and white muscle fibres there is a transition in myosin composition from an early larval form (L1R and L1W respectively) to a late larval form (L2R and L2W) and then to the isoforms typical of adult red and white muscle. The transition from L1W to L2W in the deep muscle occurs very rapidly and early in larval life (between 10 and 28 days), whereas the equivalent transition in the superficial muscle (from L1R to L2R) is a gradual process beginning in fibres near the trasverse septum and spreading hypo- and epi-axially as this layer grows around the deep muscle. The definitive adult forms (AR and AW), distinguishable by the appearance of characteristic histochemical myosin ATPase activity, are present in the superficial red muscle by 80 days, but later in the deep white muscle (by 20 months), respectively. Compared to the superficial red and deep white muscle, the intermediate (pink) muscle layer first appears relatively late (80 days), but then acquires the histo- and immunohistochemical profile characteristic of the adult form much more rapidly. The mosaic appearance of the deep white muscle is first seen during larval life. However, at this stage the smaller fibres of the mosaic have a different histo- and immunohistochemical profile from that seen in the adult small white fibres, which may indicate different mechanisms of histogenesis of new fibres in the deep muscle layer in the early and adult stages. Key words Fish - Muscle development - Myosin - Immunohistochemistry - Histochemistry