title: The fine structure of the deep muscle lamellae and their sarcoplasmic reticulum in Branchiostoma lanceolatum.
abstract: The ultrastructure of the deep lamellae of Branchiostoma's trunk muscle was investigated electron microscopically using quantitative morphometry and extracellular markers. Earlier measurements by Flood [7] were mostly confirmed and further extended. The controversy about the arrangement of the sarcoplasmic reticulum (SR) was widely resolved. The extracellular space occupies only 4.0% of the whole muscle volume and is freely accessible to extracellular markers (ferritin, lanthanum). The SR consists of cisternae without tubular interconnections. It occupies 5 to 11% of the cell volume. A junctional gap of 11 to 15 nm width separates the cisternae from the surface membrane and is bridged by typical junctional feet, 11 nm long and 10 to 20 nm in diameter. A Markham rotation analysis revealed a well defined tetragonal pattern with a centre-to-centre feet distance of 34 to 38 nm. A transverse tubular system is absent in the main part of the sheet-like muscle cells. However in basal regions with double layers of myofibrils, extracellular channels surrounded by SR cisternae could be detected within the muscle cells. The results show that there are no principal differences in membranal contacts between the diads of Branchiostoma and vertebrate triads. From these data and physiological findings [23, 24] it seems unlikely that main differences exist in the mode of excitation-contraction coupling between these muscle cells.
mesh_terms: D000818:Animals; D002816:Chordata, Nonvertebrate / Q000648:ultrastructure*; D005110:Extracellular Space; D007365:Intercellular Junctions / Q000648:ultrastructure; D008854:Microscopy, Electron; D008954:Models, Biological; D009132:Muscles / Q000648:ultrastructure*; D009210:Myofibrils / Q000648:ultrastructure; D012508:Sarcolemma / Q000648:ultrastructure; D012519:Sarcoplasmic Reticulum / Q000648:ultrastructure*