36 KERATIN AND KERATIN IZATION 



per cubic centimetre in the fixed state which is the principle factor increas- 

 ing the contrast. Experiments in vitro have established that objects which 

 exhibit the greatest contrast in osmium fixed material are: phospholipid 

 (or phospholipid plus protein) membranes, compact masses of reactive 

 proteins and lipid inclusions containing unsaturated fats. Nucleic acids, 

 mucopolysaccharides and polysaccharides show little increase in contrast, 



Fig. 17. In vitro reactions of osmium tetroxide (Bahr, 1954). 

 Reaction zvith Os0 4 (appearance of black lower oxides). 



— SH peptides, proteins, enzymes. 



= = fats, waxes, lecithin, cerebrosides, vitamins, certain hormones, 



bile acids and other biological substances containing a basic 

 sterol structure. 



->■ N tert.bases co-ordination, tryptophane. 



— NH 2 in terminal positions and not salt-linked. 

 — S — sulphide sulphur, cystine, methionine. 

 OH 



— CHO * n term i na l positions and on certain carbon chain lengths. 



Certain heterocyclic compounds. 



Aromatic compounds with at least two hydroxyl groups in suitable positions: 



plant material, tanning substances. 



No reaction with Os0 4 



— COOH acid group. 

 — CH 2 — CH 2 — paraffin chain. 

 — CO— NH— peptide bond. 

 — COO-...NH 3 + salt link. 



— HS0 3 sulphonic acid group. 



R — <^ y — OH monohydroxy- (tyrosine). 



-OH halide-substituted (di-iodotyrosine). 

 I 

 Carbohydrates — Sugars and their polymers such as starch, glycogen, pectin, 

 amino sugars, heparin, hyaluronic acid, lignin. 

 Nucleic acids — Nucleotides, ribose sugars, sugar phosphates. 

 Various forms of high and low polymer DNA and RNA. 



although compact forms of nucleic acid (see below) are dense because of 

 the phosphate groups they contain. Neutral formaldehyde may give a 

 more complete fixation of proteins; it does not, however, lead to any 

 increase in density (i.e. no staining effect) and is thus not so popular as a 

 fixative since the image is less photogenic. The presence of proteins in 

 formaldehyde fixed material may be demonstrated by staining with 



