Development of the locomotory muscle of the chaetognath Sagitta. 1. Quantitative and qualitative aspects of the body and muscle tissue development within the phylum.
The Chaetognath primary muscle constitutes the main tissue of the body-wall. It is made up of four main longitudinal bands. Muscle fibres are separated from the nervous system. During muscle growth, four myogenic zones produce two types of fibres A and B, forming units. These units ordered in an epithelial-like tissue, show various kind of intercellular junctions. For a given animal, the overall number of A and B fibres units is practically the same in each band. The four myogenic areas are the centre for a two-phase production of A and B fibres which appear at a regular rate in each quadrant, by the simultaneous production of three sets of A fibres and two sets of B fibres. The former differentiate at once, the latter some time later, and always at the same moment in their lives. In this way the number of sets of A and B fibres increases during the animal's life. This hyperplasia is accompanied by the hypertrophy of the cells which make up each group. For any given animal, the ventral quadrants are thicker than the dorsal quadrants. Chaetognath locomotory muscle is a good model on which to study the development of a skeletal muscle. Its development is closely related to the hydroskeleton growth, to the animal's shape and various other constructional factors which play a central role in this very isolated phylum.
Duvert
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Université de Bordeaux 2, UFR de BBC, Laboratoire de Cytologie and URA CNRS 339, Talence, France.
Dress
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D000818:Animals; D007448:Invertebrates / Q000254:growth & development*; D008124:Locomotion / Q000502:physiology; D024510:Muscle Development; D010802:Phylogeny