Differentiation of muscle fiber types in the teleost Brachydanio rerio.

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]