THE FINE-STRUCTURE OF THE DEEP MUSCLE LAMELLAE AND THEIR SARCOPLASMIC-RETICULUM IN BRANCHIOSTOMA-LANCEOLATUM

  • 1 January 1982
    • journal article
    • research article
    • Vol. 28  (2) , 202-212
Abstract
The ultrastructure of the deep lamellae of Branchiostoma''s trunk muscle was investigated with EM by using quantitative morphometry and extracellular markers. Earlier measurements by Flood were mostly confirmed and 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 5-11% of the cell volume. A junctional gap 11-15 nm wide separates the cisternae from the surface membrane and is bridged by typical junctional feet, 11 nm long and 10-20 nm in diameter. A Markham rotation analysis revealed a well-defined tetragonal pattern with a center-to-center feet distance of 34-38 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. There are no principal differences in membranal contacts between the diads of Branchiostoma and vertebrate triads. It seems unlikely that main differences exist in the mode of excitation-contraction coupling between these muscle cells.