An intermediate type of muscle fibre in Xenopus laevis

Abstract

AMPHIBIAN skeletal muscle is generally regarded as being composed of two distinct fibre types, twitch fibres and slow fibres, and various morphological and functional criteria have been used to classify fibres into one of these two categories (for review see ref. 1). However, the results of several investigations on frog and toad, including Xenopus, muscles suggest that this is an oversimplification2–5. Smith and Ovalle6 demonstrated five classes of fibres in hind limb muscles of Xenopus laevis using various ultrastructural and histochemical criteria. The mechanical performances of fibres of two of these classes (fast, fatigue-resistant twitch fibres (type 2) and slow fibres (type 5), respectively) have recently been described in detail7. In the study reported here fibres with a characteristic microscopic appearance have been isolated from a hind limb muscle of Xenopus laevis and it is shown that they have functional properties which are intermediate between those of twitch and slow fibres. The fine structure of these fibres conforms to the description of Smith and Ovalle for the ‘type 4’ fibre, a fibre type which also can morphologically be said to represent an intermediary type.