CELL ULTKASTRUCTLRE IN MAMMALS 



However, in appl^dng an additional stain of 

 heavy metal to the thin sections, as for in- 

 stance phosphomolybdic acid, the large 

 glj^cogen particles also become visible in liver 

 cells. 



Fibrillar Structures 



Fibrillar structures can easily be resolved 

 with the electron microscope. The}^ may be 

 located within the cell cytoplasm as is the 

 case with myofilaments, tonofilaments, and the 

 neurofibrils, or they are found at the outer 

 surface of the cells or in the interstitial 

 space, here recognized as collagen, reticular 

 fibers and elastin. 



Intracellular. Striated muscle filaments. 

 The most evident myofilaments are found 

 in the striated skeleton and heart muscle 

 cell (Fig. 11). Here they occupy the main 

 portion of the cell oriented longitudinally, 

 and they represent the contractile elements 

 of the muscle cell. The myofilaments extend 

 along the whole length of a sarcomere which 

 is the structural, repeated unit of the muscle 

 fiber. The thickness of the individual myo- 

 filament varies within different areas of the 

 myofibril, with its smallest diameter related 

 to the area of the I-band and the H-band, 

 and its largest diameter within the Z-band, 

 S-band, and M-band. In a stretched muscle 

 fiber, presumably corresponding to the re- 

 laxed position, the mean diameter of the 

 myofilament is about lOOA in the H-band, 

 whereas the same value for the M-band is in 

 the neighborhood of 150A. During muscle 

 contraction, the diameter of the individual 

 myofilament increases about three times as 

 compared to its stretched diameter. It has 

 been proposed that each myofilament in 

 turn is composed of three subunits. Each 

 subunit consists of rows of rodlets measuring 

 20A in length. The subunits are assumed to 

 represent cables of supercoiled a-helices of 

 protein molecules. 



The main constituents of the myofilaments 

 are the proteins actin and myosin. As yet, it 



has not been convincingly proved what part 

 of the myofilament represents the actin and 

 what represents the myosin. It has been sug- 

 gested that actin is represented by one set 

 of filaments and myosin by another set. 

 There is also a num})er of theories about 

 how the contraction occurs from a structural 

 point of view. Further extensive investiga- 

 tions are needed until a definite solution is 

 arrived at regarding the striated muscle. 



Smooth muscle filaments. In the smooth 

 muscle cell, the contractile elements are not 

 as easily demonstrated as in the striated one 

 (Fig. 12). This is particularly true in the 

 smooth muscle cells of the blood vessels. 

 Although it is well known that these cells 

 do contract, the cytoplasm is remarkably 

 devoid of any fibrillar structures. However, 

 in the smooth muscle cells of the small 

 bronchi and bronchioles of the lung as well 

 as in those of the intestinal wall, a longitud- 

 inal striation can more readily be seen. The 

 diameter of the filaments here average 150A. 

 The length of each filament is more difficult 

 to determine and it seems that it does not 

 extend for a length of more than half a 

 micron. The difference in ultrastructure be- 

 tween the striated and the smooth muscle 

 filaments seems to reflect the difference in 

 their function, since it is well known that the 

 smooth muscle cell has a much slower rate 

 of contraction than the striated one. 



Tonofilaments. In the cells of the epidermis 

 an elaborate system of tonofibrils crisscrosses 

 the cytoplasm. The tonofibrils are well dis- 

 tinguished in the light microscope. Their 

 ultrastructure is characterized by an abun- 

 dance of .small tonofilaments, oriented longi- 

 tudinally to the axis of the tonofil^ril (Fig. 

 13). Each tonofilament has a thickness of 

 about 190A and an approximative length of 

 0.5 micron with seemingly tapered ends. The 

 filaments have a light core and an electron 

 dense wall, the latter with a diameter of 

 about 70A. It has not as yet been possible to 

 determine whether the tonofilaments are 

 spindleshaped or slightly twisted around 



105 



