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

Dissolution of the contractile apparatus derived from larval muscle occurs when pupal morphogenetic movements are complete, but a residual myofibre remains. Synapses with nerves are retained by degenerating myofibres, although the nerves are not structurally normal. Within the myofibre, the appearance of several types of unit membrane aggregate is coincident with the disappearance of normal sarcoplasmic reticulum. One type of aggregate consists of stacked cisternae, separated by a gap of 300 Å which is traversed by a three-dimensional connecting lattice of regularly arranged rods. These latticed cisternae appear to be derived from sarcoplasmic reticulum, and they interconnect with smooth-surfaced cisternae. Structures akin to latticed cisternae have been described by others in a variety of cells. Of particular interest is a report of their formation as a result of denervation in rat muscle. This report, coupled with the evidence presented here, suggests that latticed cisternae are indicative of changed nervous stimuli in degenerating muscle. Changes in the membrane systems are rapidly followed by resorption of the myofilaments. Thick myofilaments disappear first, followed by thin myofilaments, with the eventual disappearance of Z-discs completing the dissolution of the sarcomere. No evidence was obtained for the segregation of myofilaments in membrane-bound vacuoles, or for the presence of lysosomes in muscle at this time. It is suggested that these observations are consistent with theories developed by others for retention of protein structure, rather than disassembly into amino acids, at metamorphosis in insects. Formation of surface blebs is correlated with loss of organelles to the haemolymph. Morphological evidence supports the proposition that coated vesicles are involved in exchange of proteins with the haemolymph, shortly before adult myofilaments form. Insect myoblast fine structure is described. The bipolar shape of these cells is shown to be associated with an oriented fascicle of microtubules. Attention is drawn to the plasma membrane of the myoblast, which bears folds at the points where flexion of the cell can be demonstrated by time-lapse microscopy. Myoblasts do not contain recognizable microfilaments other than microtubules. Fusion of free-floating myoblasts with the residual myofibre is demonstrated by light and electron microscopy, and the stages of fusion are described. Ultrastructual manifestation of a cell recognition process was not detected. The process of multinucleation of developing muscle cells in insects is directly comparable with myoblast fusion in vertebrate cells. However, in Calliphora the resulting myofibre contains two classes of nucleus, easily recognizable on basis of size. The structure of these nuclear classes, and the role played by each in adult myofibril formation, is investigated in an accompanying paper.