LAMELLAR SYSTEMS 



133 



sistently found in most preparations. In the axonal region, close 

 attachment, or actual fusion, of the myelin layers with the mito- 

 chondria is occasionally observed. Unfortunately, this exceptional 

 degree of structural integrity is confined to thin specimens and to 

 peripheral regions of larger tissue blocks. The remaining areas show 

 unmistakable signs of ice-crvstal formation, and all transitions can 

 be found between partial disruption of the myelin sheath structure 

 and the zones of excellent preservation. This constitutes a serious 

 limitation of all low-temperature techniques for normal histological 



— 35A- 

 — A3 — 









I — 58 

 ■87^ 



PS 



i Hi 



© 



1000 A 



^ 



220 



HlO 

 -73^ 

 -AA A 



Fig. 7. Myelin sheath segment from transverse thin section of frog optic 

 nerve, showing regular fine structure (arrow) in dense and intermediate regions 

 of concentric layers. Nerves equilibrated in glycerol before rapid freezing in 

 liquid helium II. Osmium-cryofixation preparation. X380,000. 



Fig. 8. Low-angle x-ray diffraction patterns, showing the effects of glycerol 

 treatment on the myelin sheath structure. (Above) Pattern of fresh giant toad 

 sciatic nerve. (Below) Pattern of giant toad sciatic nerve after equilibration 

 with 30 per cent glycerol in Ringer solution (24 hr), showing swelling of struc- 

 ture. Upon rapid freezing of the glycerol-treated nerve with liquid nitrogen or 

 liquid helium I (—269° C), followed by thawing, a substantially unaltered 

 x-ray diffraction pattern was obtained, in contrast to the untreated nerve, in 

 which marked modifications of the pattern were observed, indicating a partial 

 breakdown of structure. (Experiments carried out in collaboration with Dr. J. B. 

 Finean in 1957.) 



