STUDIES ON THE CROSS-STRIATION OF THE INDIRECT FLIGHT MYOFIBRILS OF THE BLOWFLY CALLIPHORA

Abstract

Cross-striation in the indirect flight myofibrils of the blowfly Calliphora was investigated by phase contrast and polarized light microscopy. Changes in the pattern of striation when the fibril changes its length or when materials are extracted from it are described. The pattern at rest-length in situ was determined in flies killed in the vapor of osmium tetroxide; all other observations were made on unfixed isolated fibrils. Contraction (which does not exceed 8% rest-length) was seen in either fresh fibrils or glycerol-extracted fibrils treated under the microscope with crude muscle extract or with adeno-sine triphosphate (ATP). Under certain conditions fresh fibrils elongated to 104% rest-length when treated with muscle extract or ATP, and over the range 98-104% rest-length an alternation of contraction and elongation was obtained. The process of glycerol extraction brought about elongation to 104% rest-length. Both fresh and glycerol-extracted fibrils could be stretched irreversibly beyond 104% rest-length; stretched zones then appeared on either side of the Z line, but no other parts of the sarcomere were affected. Over the range 90-104% rest-length, the changes in band pattern were qualitively the same as those elsewhere described by the author and H.E. Huxley in rabbit psoas muscle as the I bands disappeared the H zones closed up; no evidence was found that the A band changes its length or that the A substance migrated. Results of extracting materials from sarcomeres at different lengths led to the conclusion that there are 3 main components, as in rabbit psoas fibril: (a) the A substance which does not change its position as the fibril changes its length; it can be extracted by the same procedures as remove myosin from the rabbit sarcomere; (b) a material which extends from the Z lines to the borders of the H zone and moves inward during contraction and outward during elongation; it can capture rabbit myosin from solution and form with it a contractile system and it is thought to be actin; (c) a "backbone" or stroma bearing Z and M lines.