208 



EBBA ANDERSSON 



In this investigation, it has not been possible to 

 observe the two sets of myofilaments described by 

 Huxley (4). An explanation for this mysterious 

 discrepancy might be that we have studied muscle 

 tissue that has been fixed in the fresh state, and have 

 reduced the various stages in the preparatory tech- 

 nique to a minimum. Huxley investigated glycerin- 

 ated muscle tissue. In such material, we have 

 observed a splitting of the A-band part of the myofi- 

 lament into thinner branches, each of which shows a 

 folded or spiralized structure. 



The interconnecting bridges are similar to those 

 described by Hodge (3) in dipteran flight muscle 

 but are thinner and more numerous in vertebrate 

 muscle. 



With such contradictory results as those presented 

 here and the observations and interpretations of 



Huxley and Hanson (2, 4, 5), it seems justifiable 

 not to consider the morphologic background for 

 the contractile process as definitely unveiled. 



References 



1. Andersson, Ebba, Intenmt. J. Ultrastructiire Research, 



under preparation (1957;. 



2. Hanson, J. and Huxley, H. E., Symposia Soc. Exptl. 



Biol. 9, 228 (1955). 



3. Hodge, A. J., /. Biophys. Biocliem. Cytol. 1, 361 (1955). 



4. Huxley, H. E., Biochim. Biophys. Acta 12, 387 (1953). 



5. Huxley, H. E. and Hanson, J., Nature 173, 973 (1954). 



6. Sjostrand, F. S., Science Tools 2, 25 (1955). 



7. — Exptl. Cell Research 10, 657 (1956). 



8. Sjostrand, F. S. and Andersson, Ebba, Exptl. Cell 



Research 11, 493 (1956). 



9. — , Internat. J. (JItrastnutiire Research, under prepara- 



tion (1957). 



The Tubular System in the Striated Muscle Cell 



Ebba Andersson 



The Laboratory for Biological Ultrastructiire Research of the Department of Anatomy, 



Karoliiiska Institutet, Stockholm 



Already at the end of the 19th century Retzius (2) 

 found in his material of gold-impregnated muscle 

 tissue a network in the sarcoplasm between the 

 myofibrils. Until recently rather little attention has 

 been paid to the structure and function of this 

 sarcoplasmic detail in the muscle cell, but with the 

 development of the cell studies by means of electron 

 microscopy this network has gained new interest. 

 Bennett and Porter (I) have among others included 

 this component in the rather all-round endoplasmic 

 reticulum. 



The muscle material investigated is mainly taken from 

 skeletal muscle from frog and mouse. The material is 

 fixed /// situ in buttered isotonic osmium tetroxide solu- 

 tions. Some material was stained with phosphotungstic 

 acid dissolved in 70 % ethanol after the fixation. The 

 preparations were embedded in a //-butyl-methyl metha- 

 crylate mixture and sectioned with Sjostrand ultra- 

 microtomes. The micrographs were taken with a RCA 

 EMU 2c electron microscope. 



In a longitudinal section (fig. 1) the sarcoplasmic 

 component is recognized as tubes passing in between 

 the myofibrils. In the A-band region the tubes are 

 oriented parallel to the axis of the myofibrils. In 

 the 1-band region they run in various directions. 



partly perpendicular to the long axis of the myofi- 

 brils. In most of the preparations the tubes are dilated 

 in the I-band region. 



In a cross section (fig. 2) the tubes are cross- 

 sectioned in the A-band area and more or less 

 obliquely or longitudinally sectioned in the I-band 

 area. 



The diameter of the undilated tubes is 250-300 A. 

 The total thickness of the bounding membrane is 

 about 60 A and is triple layered with the layers 

 measuring 20 A in thickness. This is observed 

 most easily in material stained with phosphotungstic 

 acid. The tubes have a very intimate topographic 

 relation to the myofilaments. In phosphotungstic 

 acid stained material the bounding membrane of the 

 tubes seems to be in direct contact with the myofila- 

 ments. 



The relationship of the tubes to the mitochondria 

 is very intimate. Any definite direct continuity be- 

 tween tubes and mitochondria has, however, never 

 been observed. For the analysis of this relationship 

 the mouse muscle has been used with its large number 

 of mitochondria. 



Fig. 1. Longitudinal section from frog skeletal muscle, fixed in osmic acid. The lubes are longitudinally sectioned in the 

 darker A-band region and more or less cross-sectioned in the lighter I-band region. In the lower right part a longitudi- 

 nally sectioned mitochondrion with its triple-layered outer and inner membranes. Magnification 32,000. 



Fig. 2. Cross section through skeletal muscle from frog. Osmium fixation. The section has passed through both A 

 and I-band regions. In the I-band region the tubes are somewhat dilated. In the A-band region to the right a cross- 

 sectioned mitochondrion. Down in the lower part is the sarcolemma. Magnification 36,000. 



