The fine structure and mechanism of breakdown of larval intersegmental muscles in the blowfly Calliphora erythrocephala

Abstract

The intersegmental muscles of Calliphora are composed of an array of thick and thin myofilaments, in which an average of ten thin myofilaments surrounds each thick myofilament. The Z-disks are perforated, and both thick and thin myofilaments pass through the perforations when the muscle is fully contracted. During puparium formation the intersegmental muscles contract only partially, so as to bring the ends of the thick myofilaments into contact with, but not to penetrate, the Z-disk. It is shown that invasion of muscles by phagocytes is under humoral control, and can be prematurely or artificially induced by injection of crustecdysone. A change in the haemocyte population induced by crustecdysone is the first morphological indication of the onset of metamorphosis. Breakdown of intersegmental muscles in Calliphora is not mediated by nervous stimuli. The signal inviting invasion of haemocytes into individual muscles is unknown, but morphological changes in the doomed muscles precede invasion. Attenuated folds of haemocyte plasma membrane penetrate muscles which possess normal contractile elements. Adjacent folds unite, isolating muscle fragments within the haemocyte cytoplasm. Soluble or colloidal elements are taken into the haemocyte by coated vesicles, which line the pockets of haemolymph between haemocyte and muscle. Coated vesicles in circulating haemocytes are capable of isolating non-proteinaceous substances such as thorium dioxide, and transporting them into the cell, where they accumulate in vacuoles. Within the phagocytic haemocyte muscle fragments are degraded in a well-developed vacuolar apparatus. Acid phosphatase activity is demonstrated in many vacuoles. The catabolic process within the vacuolar apparatus leads to the accumulation of lipid and glycogen reserves. Protein and protein+RNA vacuoles also accumulate, but the extent of degradation of macromolecules is unknown. Rupture of the haemocyte plasma membrane leads to the eventual release of all modified muscle components into the haemolymph.