KERATIN AND MOLECULAR BIOLOGY 35 



the objects visible in the light microscope in terms of these new structures 

 has also made satisfactory progress. Since this information is not yet 

 common currency, it is advisable at this point to name and describe 

 the commonly-occurring fine structural units in terms of which our 

 later accounts of cell structure will be given. These descriptions will be 

 brief and are intended to serve simply as morphological definitions. 

 Fuller accounts of the function and structure of the units will be given 

 later. 



To a degree at the present time pure morphology has out-run knowledge 

 of the chemistry and function of cell constituents. It is, for example, not 

 always possible to state with certainty the chemical nature of the materials 

 giving images in electron microscopes. To appreciate the special nature of 

 this problem it is necessary to consider briefly the preparative procedures 

 of electron microscopy. Biological material intended for sectioning is 

 first of all fixed, i.e. subjected to a chemical treatment which kills the cells 

 and converts (more or less effectively) certain of their constituents into 

 derivatives of greater physical and chemical stability. The fixed specimen 

 is then dehydrated, which adds further to its stabilization, embedded in a 

 polymer and cut into suitably thin sections. Certain of the chemicals 

 previously employed by light microscopists, such as osmium tetroxide 

 (Os0 4 ) and formaldehyde, have been found useful as fixatives by electron 

 microscopists (Palade, 1952) and others (potassium permanganate) (Luft, 

 1956) have been introduced. The most commonly used fixative is the 

 buffered (pH 7-8) solution of osmium tetroxide introduced to electron 

 microscopy by Palade and it is with the results obtained using it that we 

 are mostly concerned with here. Osmium is an element of high atomic 

 number and there is no question that much of the contrast of osmium 

 fixed material arises from the electron scattering produced by osmium 

 atoms present in the fixed specimen either as lower oxides or as compounds 

 with the organic matrix. Unfortunately for the prospects of a super- 

 histochemistry the nature of the reactions of Os0 4 with tissue components 

 is both obscure and hitherto little studied. Bahr (1954), by treating in 

 vitro a series of pure organic compounds with osmium tetroxide, 

 has shown that certain chemical groupings are able to react with osmium 

 tetroxide to produce coloured compounds. It is probably correct to 

 assume that the reaction of osmium tetroxide with a complex molecule 

 such as a protein represents the sum of the reaction with its component 

 reactive groups. Figure 17 taken from Bahr summarizes the known 

 reactivity with osmium tetroxide. Since we are interested primarily in a 

 sulphur-containing protein, it should be noted that the most reactive 

 groups in proteins are the sulphur-containing amino acids. 



A further factor of great importance in producing contrast is the com- 

 pactness of the organic substrate. This influences the amount of osmium 



