THE LARVAL DEVELOPMENT OF LATERAL MUSCULATURE IN GILTHEAD SEA BREAM SPARUS-AURATA AND SEA BASS DICENTRARCHUS-LABRAX

Fibre-type differentiation of the lateral musculature has been studied in Sparus aurata (L.) and Dicentrarchus labrax (L.) during larval dev elopment. Histochemical and ultrastructural techniques show two presum ptive muscle layers and two germinative zones of presumptive myoblasts . At hatching, myotomal muscle consists of a monolayer of thin undiffe rentiated cells near the skin (first germinative zone) overlying anoth er monolayer of small diameter fibres extending hypaxially and epaxial ly away from the transverse septum. Below this, there is a much thicke r, deep layer of fibres, generally large in diameter and polygonal in shape. The presumptive myoblasts are located between these two layers of fibres in the second germinative zone. Initially, the superficial a nd deep muscle fibres show high and low myosin ATPase activity, respec tively. Both layers grow by generating new fibres from the two mention ed germinative zones. At the end of larval life, the superficial layer changes its histochemical profile from high to low myosin ATPase acti vity and, at the same time, intermediate or pink muscle fibres can be observed by oxidative activity (the NADH-TR reaction). Morphometric an alysis shows a significant increase in mean fibre diameter during succ essive ages, as shown by the Student's t-test (hypertrophic growth). S kewness and kurtosis values of fibre diameters point to the generation of a new fibre population from the germinative zones (hyperplastic gr owth).