MUSCLE GROWTH AND MYOSIN ISOFORM TRANSITIONS DURING DEVELOPMENT OF A SMALL TELEOST FISH, POECILIA-RETICULATA (PETERS) (ATHERINIFORMES, POECILIIDAE) - A HISTOCHEMICAL, IMMUNOHISTOCHEMICAL, ULTRASTRUCTURAL AND MORPHOMETRIC STUDY

The myosin composition of lateral muscle in Poecilia reticulata from b irth to adult was studied by ATPase histochemistry and immunostaining with myosin isoform-specific antibodies. At birth the muscle consists of two layers containing developmental isoforms of myosin. In deep lay er fibres the developmental myosin is replaced by the adult fast-white isoform soon after birth. In the epaxial and hypaxial monolayer fibre s the myosin composition present at birth (J1) is replaced within 3 da ys by another (J2). In some fibres, this J2 composition is retained in the adult, but in others it is slowly replaced by the adult slow-red muscle isoform. Close to the lateral line, all monolayer fibres are al ready in transition between the J2 myosin and the adult slow-red form at birth, and rapidly complete the transition to slow-red form. These fibres, together with others generated de novo in an underlying hyperp lastic zone, form the red muscle layer of the adult. The pink muscle d evelops during the first month after birth, and by 31 days it consists of an outer, middle and inner layer. A few middle layer fibres are al ready present at birth, while the outer layer fibres first appear 3 da ys after birth. The thin inner layer is probably a transitional form b etween the middle pink and adult white types, and appears at about 31 days. A morphometric analysis showed that growth of the white muscle o ccurs principally by hypertrophy. Even at the magnification level of t he electron microscope, no satellite cells or myoblasts which could gi ve rise to new fibres were found in the white muscle, except in the fa r epaxial and hypaxial regions and only in the first 10 days. A zone o f hyperplastic growth was also found lying just under the superficial monolayer close to the lateral line, and this presumably contributes f ibres to the red and pink muscle layers.