Skeletal Muscle Myofilaments 



207 



accounts for the difference in opacity existing be- 

 tween these two bands. The diameter is about 40 A 

 in the 1-band. In the A-bands, the diameter of the 

 myofilaments is a function of the degree of shorten- 

 ing, and varies between about 60 to 140 A. The 

 contours of the myofilaments are smooth within 

 the l-bands but are irregularly zig-zag-formed in 

 the A-band. Between the thick parts of adjacent 

 myofilaments, interconnecting bridges arc seen 

 (fig. 2). The thickness of these cross connections 

 may be as minute as 20-30 A. The distance between 

 the cross bridges extending between two adjacent 

 myofilaments varies with the degree of shortening 

 from 90 A in sarcomeres which are shortened (to 

 about 50 "„ of their equilibrium length) to 250 A in 

 extended sarcomeres. 



Judging from the variations in the opacity of the 

 individual myofilaments, these seem to represent 

 rather complicated structural units. In the shortened 

 sarcomeres, there are observed transversally oriented 

 regions of high opacity alternating with less opaque 

 sections along the myofilaments. The structural 

 pattern observed in the myofilaments is rather ir- 

 regular, but shows that on shortening there is a 

 definite tendency of transversal orientation of 

 material within the myofilaments. This fact in ad- 

 dition to the tightly arranged cross-bridges gives the 

 impression of a marked transversal orientation of 

 the structural components in the shortened sarco- 

 mere (fig. 2). 



The modulation of the opacity within the myofila- 

 ments may be interpreted as due to their consisting 

 of smaller structural subunits. Direct indications 

 of such subunits may be observed, but the electron 

 staining of the myofilaments is rather diffuse, 

 which is obvious when comparing the resolution of 

 the myofilament structure with that of more dis- 

 crete structures in the pictures. In fig. 4, it is pos- 

 sible to observe in some places that the myofila- 

 ments are subdivided into smaller filamentous 

 units with an estimated diameter of 10-20 A. This 

 picture is from an extended sarcomere in a muscle 

 that was fixed in a stretched state. The thickness of 

 the A-band part of the myofilaments is only about 

 60 A as compared with 140 A in a shortened sarco- 

 mere. It seems justifiable to correlate the longi- 

 tudinal orientation of the subunits to this stretched 

 state. 



When examining cross sections (figs. 5, 6), the 

 hexagonal arrangement of the myofilaments within 

 the myofibrils is revealed (fig. 5). Cross bridges 

 connecting adjacent myofilaments are also ob- 



served. They do not pass straight over from myo- 

 filament to myofilament, but show a more irregular 

 course with nicks. This may be explained by the 

 shrinkage perpendicularly to the length of the myo- 

 lihiments that takes place during fixation and cm- 

 bedding. At the nicks, the cross bridges may appear 

 especially distinctly because they frequently are 

 thicker at these sites. It might be these nicks in 

 combination with superposition efTects in too thick 

 sections of crossing bridges that have been inter- 

 preted as cross sections of a second type of thinner 

 myofilaments interposed between the thick myo- 

 filaments within the A-band region (4). 



In high resolution pictures, it is striking to ob- 

 serve the angular shape of the myofilaments in 

 cross sections and the rather high frequency of 

 triangular cross sections. The interconnecting 

 bridges between adjacent myofilaments, in most 

 cases, extend between the corners of the triangular 

 cross sections. In addition to the triangular form of 

 the cross sections, there are other less well defined 

 forms. The triangular form seems to correspond 

 to the thickest regions of the myofilaments (fig. 6). 



Measurements of the length of A- and I-bands 

 as well as that of the sarcomeres at various degrees 

 of shortening (I) have revealed that both I- and A- 

 bands shorten with decreasing sarcomere length. 

 The change is, however, more pronounced in the I- 

 bands. As no definite new structural pattern than 

 those characteristic for the I- and A-bands appear in 

 connection with the shortening it seems justifiable 

 to conclude that the I-band parts of the myofilaments 

 structurally have changed to the organization 

 characteristic for the A-bands. 



The variations in the diameter of the A-band part 

 of the myofilaments with varying degree of shorten- 

 ing, the variation in distance between the intercon- 

 necting bridges, and the transformation of the I- 

 band part of the myofilament into an A-band type 

 of structure are interpreted as pointing to a change 

 in the organization of the individual myofilaments 

 as responsible for the shortening of the sarcomere. 

 The ultrastructure of the myofilaments indicate that 

 these filaments consist of smaller subunits with a 

 diameter of 10-20 A. These units might represent 

 thin ropes of Corey-Pauling's a-helices. The shorten- 

 ing would then be due to a folding of the compound 

 a-helices or to a change in the spiralization with a 

 shortening of the pitch of the secondary helices. 

 Assuming that there are three main subunits in 

 each myofilament would give a simple explanation 

 to their triangular cross sections (9). 



Fig. 4. Longitudinaf section through stretched muscle fiber. Indications of subunits measuring 10-20 A in some myofila- 

 ments. Notice the small diameter of myofilaments as compared to fig. 2. Magnification : 360,000. 



Fig. 5. Cross section through //;. hkeiys fcmoris of the frog. Most of the section through the A-band region. In lower 

 left corner the section cuts through the I-band region. Magnification 200,000. 



Fig. 6. Higher magnification of cross section through m. biceps femoris of the frog. Notice the considerable number 

 of triangular cross sections (indicated with arrows). Magnification 300,000. 



