PRIMARY fixatives: NON-COAGULANTS II3 



night in a solution of formaldehyde (presumably as a disinfectant). 

 In the morning he was surprised to find that it felt as hard as 

 though it had been preserved in ethanol. He then undertook a 

 systematic study of the effects on tissues, trying it at 4% on 

 various organs, including liver, kidney, the mucous membrane of 

 the stomach, and brain. He found that it hardened tissues faster 

 than ethanol and preserved their external form better, so far as 

 could be seen with the naked eye. He embedded various organs in 

 collodion, sectioned them, and found the cells well fixed and capable 

 of being dyed with haematein and synthetic dyes. It is perhaps 

 fortunate that he used collodion, because formaldehyde unmixed 

 with other fixatives gives rather poor results when tissues are em- 

 bedded in paraffin (p. 118). 



Reactions with proteins. Formaldehyde does not coagulate 

 albumin; it renders this protein not coagulable by ethanol. ^^ Un- 

 like the coagulative fixatives described in the last chapter, it 

 stabilizes gelatine gels very perfectly, so that they do not dissolve 

 in water at 37° C but retain their form and transparency. There is 

 a slow gelatinizing action on histone.**^^ Haemoglobin is retained 

 in blood-corpuscles, perhaps with conversion to methaemo- 

 globin.*^^ Fixation by formaldehyde does not affect the action of 

 pepsin or trypsin on the proteins of blood plasma. There is 

 contradictory evidence about the ability of trypsin to digest 

 collagen fixed by formaldehyde.^^^' ^^^ 



The chemistry of the reactions with proteins has been 

 thoroughly investigated. Formaldehyde reacts with the -NHg of 

 the side-groups of certain amino-acids, probably forming methyl- 

 ene bridges that link protein chains together, lysine to lysine or 

 lysine to glutamine. The reaction is slow, especially below pH 3 : 

 the greatest binding of formaldehyde occurs at about pH 7-5 to 8. 



Nucleoproteins are not coagulated; extremely minute flocculi 

 may appear eventually. DNA is not precipitated from its solu- 

 tions. ^^^ The amount of nucleic acids (DNA -|- RNA) in cells is 

 reduced by 10 to 35% by formaldehyde fixation. 



Certain enzymes are not wholly inactivated by formaldehyde. 

 The enzymes in the liver of the rat have been studied in this 

 connexion. ^^^ /3-glucuronidase retained its activity best; then 

 sulphatase, acid phosphatase, and esterase ; alkaline phosphatase 

 least well. 



Reactions with lipids. In general, formaldehyde is a good pre- 

 servative of lipids, including cholesterol and its esters, unless 



H 



