LACTATE DEHYDROGENASE IN THE RETINA OF THE GUPPY 



773 



Dhotoreceptor. They claim that this is supported by histochemical experiments 

 ihowing the complementary subcellular distribution of enzymes required for 

 glucose oxidation, and LDH, which is required for glycolysis. LDH is most 

 abundant in the régions of the cell which have no mitochondria (Lowry et 

 1956). In Lebistes, however, LDH activity is evenly distributed over the whole 

 inner segment. 



In the retina of Lebistes, most of the LDH activity, and in particular, ail of 

 jthe E-isozyme activity, résides in the cônes. The first differentiation of the photo- 

 ireceptors in the embryo is simultaneous, i.e. ail three types of cônes, and the rods, 

 jare formed simultaneously. The structure of the newly formed cônes and rods, 

 jas revealed by ordinary histological methods, is the same as in the adult eye. 

 (Muller, 1952). This seems to be reflected in the pattern of the E-bands: ail bands 

 appear simultaneously, at stage 7, and show already adult distribution of intensity. 

 Ail through the growth phase of the retina, new photoreceptors are being formed 

 at the periphery. Kryostat sections of ail stages show always intense activity of 

 E-isozymes also in thèse newly differentiated cônes. 



When sections are treated with the inhibitor, formazan deposits in the 

 cônes are less intense. This would suggest that other isozymes than E-isozymes 

 (réside in the cônes. 



Perhaps the restriction of the E-isozymes to the cônes in Lebistes is related 

 to the unique tiered arrangement of the cônes, which as such demands a functional 

 explanation. It has been suggested, but not confirmed, that this arrangement 

 should minimize, or even completely correct, the chromatic aberration; this 

 would resuit in a greatly increased acuity of the eye (Eberle, 1968). On the other 

 jhand, a double row of cônes has been observed in two tree squirrels (Sciurus 

 carolinensis leucotis and Tamiosciurus hudsonicus loquax), which show a pure 

 cone retina. (Tansley, 1961). The author suggests that this arrangement may be 

 a device to increase the sensitivity by increasing the number of photoreceptors 

 converging on to each ganglion cell; this would, however, be expected to resuit in 

 Idecreased visual acuity. 



One other teleost, Xiphophorus helleri, has been tested for LDH (and E- 

 isozyme) distribution in the eye (Whitt and Booth, 1970). The E-isozymes 

 were found predominantly in the inner segments of the photoreceptor cell and in 

 the outer nuclear layer. The authors fail, however, to difïerentiate between rods 

 i and cônes. Both Xiphophorus and Lebistes belong to the poeciliidae. Judging 

 from fig. 2 of their publication, the retina of Xiphophorus shows the same 

 unusual pattern of cone distribution as Lebistes. Thus the heavy formazan 

 deposits attributed by Whitt and Booth to the outer nuclear layer, are probably 

 j located in the ellipsoids of the inner cônes. This assumption is based on the fact 

 < that in Lebistes the nuclei of the middle cônes penetrate the outer limiting mem- 

 brane, so that the ellipsoids of the inner cônes seem to lie within the outer nuclear 



