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The trunk musculature of adult zebrafishes contains three major fiber types: adult red, intermediate, and white; and two minor populations: red muscle rim and scattered intermediate fibers. In this paper, the post hatching development of these muscle fiber types was studied by means of immunohistochemistry, using anti-myosin sera. Just hatched larvae contain two muscle fiber populations: embryonic red and white which give rise to the red muscle rim and the intermediate fibers respectively. Adult red fibers arise post hatching as a new separate population with distinct myosin properties. The differentiation of these fiber types occurs within the first four weeks after fertilization, when the adult pattern of peripheral axon bundles has become established. Differences in the muscle fiber type composition between the midbody and the tail myotomes become apparent in two month old fries. The number of scattered intermediate fibers increases from rostral to caudal, the opposite holds for the red muscle rim fibers. The red and intermediate area is triangular in the midbody; in the tail part it is stretched out along the lateral surface of the myotomes. These changes are considered as adaptations to improve the efficiency of the swiming performance. [ABSTRACT FROM AUTHOR]

In this study we tested the effects of immobilization and of spinal cord lesion on the differentiation of muscle fiber types in the myotomal muscle of the zebrafish. We found that the differentiation of embryonic red and white muscle fibers occurs in immobilized as well as in denervated myotomes of zebrafish embryos. Intermediate and adult red muscle fibers are not formed in denervated myotomes of larvae. Adult zebrafishes with spinal cord lesions in the anal region show abnormal undulating movements in the rostral part of the body. This change in muscle activity correlates with an increase in the number of intermediate scattered fibers. We conclude that direct contact with spinal cord motorneurons is not necessary for the differentiation of embryonic red and white fibers. The differentiation of intermediate muscle fibers is under extrinsic control. The factors that determine the formation of adult red fibers are not yet clear. [ABSTRACT FROM AUTHOR]

A light-microscopic immuno- and enzyme histochemical and an ultrastructural study has been made on the lateral body musculature of the teleost Brachydanio rerio in the adult stage and in early developmental stages. On basis of the myofibrillar and the metabolic properties, the following types of muscle fibers can be distinguished in the adult stage: superficial red fibers, which are slow-contracting and fatigue-resistant; and deep white fibers, which are fast-contracting and fatigue rapidly. In the white fibers a subgroup can be discerned that is similar to fast-contracting and fatigueresistant fibers of mammalian muscles. The origin of these different fiber types has been investigated in early embryonic and larval stages. All muscle cells pass through a stage in which they produce 'redlike' myofibrils that are immunologically similar to the fibrils of the slow contracting superficial muscle fibers. In all, except the superficial cells, there is a switch to the synthesis of 'whitelike' myofibrils. The 'redike' myofibrils have by then disappeared. The cause of this switch is not yet clear; mechanical factors may be involved, but a neuronal influence is unlikely. The adult fiber types develop as late as in the free-living larval stage from a common embryonic fiber type first by a differentiation in myofibrillar properties and subsequently by a differentiation in metabolic properties. [ABSTRACT FROM AUTHOR]