Ultrastructural changes during transition of larval to adult intersegmental muscle at metamorphosis in the blowfly Calliphora erythrocephala

Abstract

ABSTRACT A residual myofibre resulting from autolysis of larval muscle, and from myoblast fusion, has been described in an accompanying paper. This report traces the reorganization of such a myofibre during development of contractile adult muscle at metamorphosis. Microtubules remain in the residual myofibre, but they are not oriented with respect to the fibre axis. Organization of microtubules into an array precedes myofilament formation, and this array is oriented with respect to the fibre long axis. The distribution of microtubules is ordered but not precise, and statistical evidence is presented to show that there is a preferred separation distance of 800 Å between adjacent microtubules. Possible mechanisms for control of this distribution are discussed. It is suggested that long-range electrostatic forces may be involved, rather than structural cross-bridges. Coated vesicles occur on the plasma membrane, but are not obviously associated with sarcomere organization. There is no morphological evidence that larval sarcomere organization persists in residual myofibres, and the first indication of adult sarcomeres is the development of periodic electron dense deposits at the periphery of the myofibre. The electron dense deposits develop into Z-bodies and define the adult sarcomeres. Finely filamentous material is associated with the Z-bodies, but the nature of this material is obscure. The filaments have been termed ‘initial filaments’ by other authors working on developing muscles, but an anatomical similarity with tertiary ‘ultrathin’, ‘residual’, or ‘C’ filaments, described in contractile muscle, is pointed out. The first-formed thick primary myofilaments are of reduced diameter, as in certain other insect and rat muscles. The orientation of developing myofilaments is related to the pre-existing microtubule array, which appears to serve the function of ‘scaffolding’. The ratio of myofilaments to microtubules slowly increases, but microtubules remain in adult muscle. The presence of a microtubule disturbs the precision of the myofilament paracrystalline array. Infoldings of the plasma membrane extend into the T-system, and develop an association with small cisternae (that have already formed) between microtubules. The association becomes a diad, and the cisternae become sarcoplasmic reticulum. Large nuclei derived from larval muscle bear many nuclear pores and a well-developed fibrous lamina, and are believed to be highly polyploid. Small nuclei derived from myoblasts by cell fusion bear few nuclear pores and an indistinct fibrous lamina, and are believed to be diploid. Changes in both types of nuclei during muscle development were followed by staining methods and autoradiography. Cessation of RNA synthesis by large nuclei was accompanied by separation of chromosomes from the nuclear membrane and eventual pycnosis, but not by detectable changes in the number of nuclear pores. In muscle preparations maintained in vitro, RNA synthesis declined in the large nuclei during the period of burgeoning of the myofilaments, but continued in small nuclei derived from myoblasts. It is concluded that control of the syncytial cytoplasm by both types of nucleus ceases in favour of small nucleus autonomy before the adult muscle becomes functional. In the adult muscle an average of ten, and a maximum of twelve, thin secondary myofilaments surround each thick primary myofilament. In strongly contracting adult muscle both classes of myofilament pass through holes in the Z-discs, and the sarcomere becomes shorter than the length of a thick filament. This ‘supercontraction’ has not been described in adult insects, although it is well known in larval dipteran muscles.