VISUAL SYSTEM: STATE OF THE ART 41 



and horizontally in the posterior pole of the lens (Duke-Elder 1958, Prince 

 1956). 



The lens of elasmobranchs has lately received much attention, with two 

 central themes: characterization of the crystallin protein fractions and 

 immunochemistry. The former relates to clinical interest in the origin and 

 treatment of cataract, and the latter primarily with systematics and 

 phylogeny of the vertebrates. However, the subjects are interrelated since it 

 is the evolution arily conservative antigenic proteins that are responsible for 

 lens cataract. 



Lens Crystallins— The ocular lens is 65% water and about 35% struc- 

 tural protein (Lerman 1969). Morner (1894) long ago showed that lens 

 proteins could be separated into soluble crystallin and insoluble albuminoid 

 fractions. Three types of crystallin were identified and assigned the Greek 

 letters a, (5 and 7 on the basis of molecular weight, number of subunits, and 

 other physicochemical characteristics. A fourth species, Delta crystallin, was 

 later isolated (Rabaey 1962). Alpha, |3, and 7 crystallin are heteropolymers, 

 oc having the greatest number of subunits. Gamma crystallin is a monomer 

 (Clayton 1974). These proteins play a major role in refraction and trans- 

 mission of light, which is the main function of the lens. Trokel (1962) 

 suggested that the spatial orderliness of intact protein fibers confers trans- 

 parency on the lens. The high concentration of refractile protein gives the 

 lens its overall high refractive index (Clayton 1974). 



Insoluble protein— Changes in relative concentration of crystallin are 

 associated with a progressive increase of insoluble albuminoid throughout 

 life. In man, normal increase in albuminoid during aging is reflected in loss of 

 lens elasticity, leading to presbyopia in middle age. Abnormal increase leads 

 to nuclear cataract formation. Lerman (1970) presented a summary of his 

 earlier studies (Lerman et al. 1968, Lerman 1969) on ontogenetic protein 

 changes in the dogfish. He reported that during aging soluble 7 protein in 

 the lens of the "dogfish" (species not given) progressively changes to insolu- 

 ble albuminoid by polymerization and formation of S-S linkages. This situa- 

 tion is similar in the rat but quite different in the lens of man: in humans, 

 the insoluble protein is derived mainly from j3 crystallin. Lerman in addition 

 listed the amino acid content of a, (3, 7, and albuminoid fractions of the 

 dogfish lens. 



Mehta and Lerman (1970) isolated j3 and 7 crystallin from the insoluble 

 fraction of the dogfish lens. Alpha may also be present, but it is albuminoid 

 perhaps "masked" by incorporation into aggregates. 



Zigman etal. (1970) measured a 240-fold increase in weight of total insolu- 

 ble lens protein (TIP) beween young "dogfish" sharks 60 cm in total length 

 and older animals of 150 cm. Lens weight in the experimental animals in- 

 creased by only tenfold. Thus the lens of the young dogfish contained 1% 

 TIP by weight while that of the adults contained 25% TIP. Most of the shift 

 was toward urea-soluble albuminoid. Again, ontogenetic changes in the lenses 

 of both rat and dogfish were remarkably similar. 



