158 MACROMOLECULAR COMPLEXES 



Wald, G. 1958. Photochemical aspects of visual excitation. Exptl. Cell Research 

 Suppl. 5: 389. 



WoLKEN, J. J. 1958a. The chloroplast structure, pigment, and pigment-protein com- 

 plex. In The Photochemical Apparatus: Its Structure and Function. Brookhuven 

 Symposia in Biol. No. 11: 87-100. 



WoLKEN, J. J. 1958b. Photoreceptor structures. Ann. N. Y. Acad. Set. 74: 164-181. 



Yamada, E., K. Tokuyasu, and S. Iwaki. 1958. The fine structure of retina studied 

 with electron microscope. II. Pigment epithelium tmd capillary of the chorio- 

 capillary layer. /. Electronmicroscopy (Chiba) 6: 42-46. 



DISCUSSION 

 A. C. Giese, H. Ferndndez-Mordn 



Dr. Giese (Stanford University): For what biological problems other than 

 the ones you have mentioned are the techniques you describe likely to be 

 especially useful? 



Dr. Fernandez-Moran: The described cryofixation and related low- 

 temperature preparation techniques have been used primarily for the study of 

 biological lamellar systems and of nuclear structures. Although they are still 

 in an early stage of development, the following fields of application appear to 

 be the most immediate ones: 



(1) The study of nuclear stmctures. At low temperatures, and with the 

 described techniques, nuclear stiuctures appear to be better preserved than 

 by conventional procedures. Combined staining, selective extraction, and 

 enzymatic digestion can be carried out on these sections which have been 

 subjected to a minimum of chemical fixation. 



(2) Of particular interest are the autoradiography techniques. The pre- 

 liminary attempts to increase the resolving power of autoradiography by using 

 tritium-labeling in connection with electron microscopy should be carried 

 further. If successful, they would permit detection of radioactive tracers at the 

 macromolecular level; that is to say, at a resolution of 200 to 400 A, and per- 

 haps less in the case of track formation. This would be particularly useful in 

 connection with radiotracer studies of nucleic acid components in bacterio- 

 phages and viruses for the contact radiography method. A collateral investi- 

 gation could, of course, also be carried out with dilterential ultracentrifugation 

 and radioautography studies. In this case, the pellet itself would be sectioned, 

 and radioautography performed, thus considerably increasing the scope of this 

 technique, which has already proved so powerful. 



Beyond these, there are, of course, numerous other applications. The main 

 value of the new techniques derives from the fact that we are now in a position 

 to considerably extend the applications of high-resolution electron microscopy 

 in the study of biological tissues. Moreover, low-temperature stage observa- 

 tions permit us to enter a domain which has hitherto remained unexplored. 

 Taking into consideration that practically all observations published to date 

 have been made with the specimen heated to relatively high temperatures 

 within the microscope, the value of being able to cool it down becomes evi- 

 dent. Specific examples would be: 



