Organic Substances 93 



b) Cholesterol esters. 



c) Steroid hormones. 



F. Group of chemically unidentified lipid pigments, to be dealt with in 

 greater detail in the section on pigments (lipofuscin, ceroid). These 

 substances appear to be complicated polymerization products of 

 unsaturated fatty acids. Their solubility in lipid solvents is variable; 

 some of them will resist even embedding in paraffin. 



It should be made clear that almost every group of sub- 

 stances mentioned includes a vast array of individual com- 

 pounds. The theoretically possible number of glycerids, and 

 phosphatids, according to the laws of permutation, runs into 

 the thousands because of the large variety of fatty acids 

 which can combine. In addition, practically none of the in- 

 dividual groups occurs singly within morphological struc- 

 tures (droplets, mitochondria, etc.) but almost always as 

 admixtures with other groups and in highly variable ratios. 



The physical properties of pure substances may be 

 markedly modified by admixtures. This is a very important 

 point because some of the most widely used histochemical 

 methods for the demonstration of lipids, depend on the 

 purely physical phenomenon of the solubility of certain dyes 

 in them. 



The two most important features of lipid substances affect- 

 ing their stainability by dyes are their melting points and 

 the presence or absence of hydrophilic groups ( especially if 

 the dye is applied in an aqueous solution ) . It is obvious that 

 in the case of a solid, hydrophobic lipid a partition equilib- 

 rium between the lipid and the aqueous ( or dilute alcoholic ) 

 phases can never be established, simply because the dye can- 

 not penetrate the lipid particle to any appreciable depth. On 

 the other hand, a lipid of oily consistency or one possessing 

 hydrophihc groups may be stained through and through. All 

 transitions between these two extremes are possible. It is 

 easy to see that admixtures lowering the melting point or im- 

 parting a certain degree of hydrophilia will enhance the 

 staining of the particles in bulk, whereas coating of other- 



