44 VISION 



from 17% to 5% during aging. Gel filtration of dogfish CPP revealed frac- 

 tions of three crystallins, a and j3 contributing about 20% and 7 the re- 

 mainder. Dogfish CPP is thus similar in physicochemical properties to its 

 mammalian counterpart. The authors suggested that urea retention is an 

 environmentally tuned factor maintaining clarity at the lens at less than 

 10° C. However, one must inquire into factor(s) maintaining lens clarity in 

 the majority of fish which do not retain urea but possess transparent lenses 

 even at less than 0° C. 



Calhoun and Koenig (1970) confirmed the presence of CPP in the dialysed 

 lens of the hammerhead shark, Sphyrna diplana. 



Immunochemistry— Perhaps the most interesting work on lens 

 proteins has been the use of their antigenic properties to estimate the evolu- 

 tionary history of the vertebrates. Basic to these studies is organ specificity 

 first descirbed by Uhlenhuth (1903). He injected rabbits with small amounts 

 of bovine lens proteins, which resulted in antibody production specific to 

 the injected proteins. Serum antibodies from such "immunized" rabbits re- 

 acted not only with bovine lens proteins but also with lens material from 

 many other vertebrates. This Uhlenhuth called organ specificity, in contrast to 

 species specificity seen for example in blood serum proteins which cross- 

 react within a few closely related species (Manski et al. 1965). This means 

 that the blood proteins of unrelated animals are quite different, while the 

 lens proteins can be identical. In addition, lens antigens (proteins) are con- 

 sidered to be restricted to the lens. This is known as tissue specificity, 

 although not all authors so distinguish between species, organ, and tissue 

 specificity (Waley 1969). 



Organ specificity is taken to mean that lens proteins shared by two or 

 more modern vertebrate classes must have originated in a common ancestor. 

 This statement forms the basis of the study of vertebrate phylogeny on a 

 molecular level (Manski et al. 1965). Early ontogenetic isolation from the 

 rest of the body combined with specific optical constraints and phylogeneti- 

 cally rapid perfection may explain the apparently slower and conservative 

 evolution of lens proteins. 



The concept and supporting data of tissue specificity have recently been 

 criticized (Clayton et al. 1968). The authors listed 28 publications 

 (1944-1965) in which antisera prepared against lens crystallins reacted not 

 only with eye tissue but also with extraocular tissue such as liver, kidney, 

 and muscle. Clayton et al. do not distinguish between organ and tissue 

 specificity, using both terms synonymously in the text. The result of their 

 study confirms that the lens contains a mixture of proteins with perhaps a 

 few specific, while the remainder are widely distributed throughout the 

 body. Thus, tissue specificity is viewed as a tendency toward a specific com- 

 bination and concentration of antigens rather than a unique group of pro- 

 teins restricted to the lens. 



Manski and his colleagues (Manski and Halbert 1965, Manski et al. 1965, 

 1967a, 1967b, 1967c) have investigated in detail the antigenic relationships 

 of lens proteins among a host of vertebrate species. Their elasmobranch work 



