46 VISION 



assume, based on Lerman's early work, that the "dogfish" was Mustelus 

 canis. 



Lerman et al. (1962) described three types of RNA from the dogfish lens: 

 insoluble or albuminoid, not found in any other body tissues; soluble or 

 SRNA; and microsomal. Little RNAase activity was reported. The pattern of 

 aging relative to changes in RNA was similar in dogfish, skate (species not 

 given), rat, and rabbit: SRNA and microsomal RNA remained unchanged, 

 while insoluble RNA increased in older animals. Carbohydrate metabolism, 

 however, appeared to be very much different in the shark. Intermediary 

 metabolism in the lens of mammals consists of the well-known pentose 

 phosphate pathway, glycolysis, and the less universal sorbitol pathway (van 

 Heyningen 1969). In the shark, glycolysis is the major pathway of lens 

 glucose oxidation throughout life. In contrast, the pentose phosphate path- 

 way is most active in young rats but diminishes in importance during aging. 

 Paradoxically, the RNA pattern in carbohydrate metabolism of the skate 

 lens was more closely akin to that of the rat than that of the dogfish. 



Lerman and Fontaine (1962) confirmed the similarity of dogfish and rat 

 RNA but reported that insoluble RNA increases tenfold in the dogfish dur- 

 ing aging and only twofold in the rat. In addition, labeled (C 14 ) leucine and 

 P 32 incorporation studies indicated a marked decrease in turnover of 

 albuminoid RNA during aging. 



Lerman et al. (1963) determined composition and nucleotide ratios of 

 lens RNA in rat and dogfish by paper chromatography and subsequent 

 spectrophotometric analysis. Results again indicated a close similarity 

 between rat and dogfish: purines (adenylic and guanylic acid) were present 

 at about 30 moles percent while pyrimidines (cytidylic and uridylic acid) 

 were found to be 20 moles percent. The composition of lens and kidney 

 RNA was also very similar. 



Lerman et al. (1965) confirmed the RNA base ratios given above and 

 reported that the albuminoid RNA molecules were relatively small, with a 

 molecular weight of less than 50,000. The albuminoid RNA apparently 

 derived from soluble RNA. Results of a so-called pulse labeling experiment 

 involving uptake of C 14 uracil in an effort to identify messenger RNA were 

 inconclusive. Uptake in any case was exceedingly slow— 10~ 6 nmol/h. Meta- 

 bolic inhibitors (ouabain, cyanide) apparently facilitated passage of labeled 

 uracil through the lens capsule. Finally, the authors remarked on the simi- 

 larity of dogfish and mammalian lenses, in that both have a single layer of 

 subcapsular epithelium along the anterior lens surface extending slightly 

 beyond both equators. 



The Retina 



In strictly visual terms, the retina, an embryological extension of the brain, is 

 the most important structure of the vertebrate eye. The other ocular com- 

 ponents can be thought of as accessory structures. It is in this nearly trans- 

 parent tissue that transduction of a photic stimulus into an electrochemical 

 signal takes place. How that signal is produced and processed and in what 



