2 PROTEINS 557 



Electron microscopy. The electron microscope provides a means of 

 checkine the conclusions derived from the results of indirect methods. 

 WoLPERS (1944) and Hall, Jakus and Schmitt (1946) find the 

 following micrographs of striated muscle fibres (Fig. 177): the Q 

 segment is dark and interrupted by a cross-band M, whilst the I 

 segment is clear; i.e., there is a denser packing of protein in Q and 

 a much looser arrangement in I. The most surprising result is the 

 complete blackness of the Z zone in contrast to the lack of electron 

 scattering in the adjacent N zones (Fig. 177 above). In that part we 

 must assume the presence of heavy atoms and, as Caspersson and 

 Thorell (1941) have found more nucleic acids in the semi-isotropic 

 sections of the fibres, it is likely that phosphorus, besides metallic 

 cations like potassium, is accumulated in the Z zone. It might also 

 be possible that the Z zone has a special adsorbing power for heavy 

 metals, since osmium fixation (Wolpers, 1944) or phosphotungstic 

 staining of the fibres has been used in the previous treatment. In 

 contracted muscle fibres much electron scattering material is found 

 in the I band (Fig. 177 below). F. O. Schmitt (1950a) assumes that 

 on irritation there is a migration of Q-substance into the I band, 

 causing the much discussed reversal of striation. 



The microscopic myofibrils consist of parallel submicroscopic 

 microfibrils of 100-150 A diameter. Like the myofibrils, these micro.-, 

 fibrils run straight through the segments and across their border lines. 

 For this reason earlier attempts to explain the weak optical anisotropy 

 of the I bands by a disorientation of submicroscopic elements must 

 be discarded. The microfibrils produce the X-ray interferences of both 

 actin and myosin ; hence they are considered to consist of actomyosin 

 (AsTBURY, 1947/49). X-ray diffraction discloses a long-range axial 

 period of 400 A and a short-range spacing of 27 A, while in F-actin 

 54 A has been found (Schmitt, 1950a). 



In palladium shadowed electron micrographs Rozsa, Szent- 

 Gyorgyi and Wyckoff (1950) offer evidence of the incrusting 

 materials in the myofibrils. They find a heavy incrustation in the Z 

 and M zones. Further, the whole Q band is rich in interfibrillar 

 substance except two narrow zones, called H zones, adjacent to the 

 M stripe which intersects the Q segment. Unexpectedly the I bands, 

 with the exception of the Z stripe, are free from such substances. The 

 authors consider the microfibrils to represent pure F-actin and discuss 



