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  • Title: [Evolution of muscles in Nereidae (Annelida polycheta) during epitoky. I and II. Atokous and epitokous longitudinal fibres (author's transl)].
    Author: Wissocq JC.
    Journal: Arch Anat Microsc Morphol Exp; 1978; 67(1):37-62. PubMed ID: 747394.
    Abstract:
    The longitudinal fibres from atokous Nereis belong to the double oblique striation type. Their thick myofilaments, paramyosinic (about 145 A periodicity) are arranged in an hexagonal lattice, well preserved after a glycerol treatment. 9 to 13 thin filaments form an orbit around one thick filament (ratio: 6-7/1). The contraction occurs in two processes: a sliding filament mechanism, and a shearing mechanism. This second mechanism consists in a parallel shifting of thick filaments, arising their overlap and decreasing their degree of stagger; it induces the increase of the myofibrillar (A-bands) oblique angle from 10-12 degrees in isolated glycerol extracted fibres, to 35-38 degrees in the same fibres after contraction by the action of ATP-solution. The longitudinal fibres from epitokous Nereis or Heteronereis are characterized also by a double oblique striation. The structural aspects of the contraction are similar to these ones occurring in atokous fibres. Nevertheless, the epitokous fibres are different from the atokous fibres by many characteristic points. Their thick myofilaments, with an hexagonous array, are thinner and shorter than these ones from atokous fibres. The contractile material is only present in the cortex. The axis of epitokous fibres is filled with numerous mitochondria, between them numerous glycogen particles have been synthetised. The sarcoplasm of the fiber's coelomic edge is often devoided of myofilaments but it is full of mitochondria and glycogen granules. The nucleus, with a voluminous nucleolus, is no more placed in the fibre axis, but in a lateral sarcoplasm containing ribosomes, glycogen and mitochondria. The important transformations of longitudinal fibres are discussed in connection with the Heteronereis locomotor behaviour, very different from this one of the atokus Nereis. Particularly, it seems that the abundance of glycogen and mitochondria in epitokous muscles allows a high contraction frequency. The decrease of the diameter and length from the thick filaments should be set in relation with the augmentation of the contraction's swiftness. These results are very similar to these ones occuring in Syllidae with the stolonization mode of reproduction. The Syllis stolonial fibres present the same characteristics than the Nereis epitokous fibres.
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