ELECTKON iVIICKOSCOPY 



Fig. 9. Section of human skin. The edges of the 

 elastic tissue have embedded some of the neigh- 

 bouring collagen fibrils. Large spaces are due to 

 the high fluid content of the tissue, Osmium tet- 

 roxide fixation. Embedding medium removed. No 

 shadowing. X 10,000. 



the low magnification of Fig. 21 they are 

 clearly distinguishable. But they are com- 

 posite crystallites, so that in addition to the 

 apparent detail produced by the physical 

 phenomena already mentioned, there are 

 the real sub-boundaries of a column of 

 bricks. This is demonstrated in photographs 

 such as Fig. 10, taken by R. W. Fearnhead 

 at the London Hospital, in which total re- 

 flection of many of the sub-units can be seen 

 in one photograph but not in the other. The 

 angular difference of these two photographs 

 was 3°. The width of crystalUtes is of the 

 order of 500A (they are usually thinner) and 

 total lengths are upwards of 2000 A. 



In cartilage and bone the crystallites are 

 first laid down in a haphazard manner bear- 

 ing no relationship to the direction of the 

 collagen fibrils. This has been observed both 

 directly (R. A. Robinson and D. A. Ca- 

 meron, J. Biophys. Biochem. Cytol. Suppl. 

 2(4), 253, 1956) and indirectly, using other 

 techniques (G. Wallgren, Acta Paediatrica, 

 Suppl. 113). In Oxford, a combination of 

 electron microscopy and electron diffraction 

 has shown that in newly calcified dentine the 

 orientation of the hydroxyapatite crystal- 



FiG. 10. Crystallites from dental enamel. Total reflection of electrons occurs in different regions in 

 the two photographs, taken at an angle of 3°. X50,000. Photographed by R. W. Fearnhead at the London 

 Hospital. 



282 



