EXPERIMENTAL MAMMALIAN TERATOLOGY 239 



of these rats had characteristic patterns of malformations, and that the array of mal- 

 formations could be modified greatly by adding vitamin A to the diet at certain stages 

 of pregnancy. For instance, when vitamin A was added to the diet before the thirteenth 

 day of gestation, virtually no ocular malformations occurred; but when it was added 

 after the fifteenth day, there was no reduction in the number of ocular defects. Mal- 

 formations of the aortic arch, on the other hand, were prevented by addition of vitamin 

 A before the twelfth day, but cardiac defects were not prevented when the vitamin 

 was added as early as the tenth day. 1394 It could be inferred, therefore, that certain 

 processes concerned with organogenesis required more vitamin A than others (since 

 some organs develop normally in embryos from unsupplemented animals) and that the 

 requirements vary from one developmental stage to another. 



The synthesis of analogs to a number of biochemical compounds made it possible 

 to refine this approach, by using an analog of a particular nutritional element to 

 produce a temporary inhibition of the activity of that element. There is some question 

 as to whether substitution of an inactive analog (which might conceivably have toxic 

 effects in its own right) is strictly analogous to a sudden deficiency of the compound 

 concerned, but the technique does provide a useful analytical tool. Nelson and her 

 group, for instance, have used a maternal diet deficient in pteroylglutamic acid (PGA) 

 supplemented with a PGA analog and succinyl-sulfathiazole (to inhibit PGA synthesis 

 by the intestinal flora) to produce malformations in rats. 940 A 36-hour period on the 

 diet followed by high levels of vitamin supplementation to terminate the deficiency 

 was teratogenic, but a 24-hour period was not. The type and frequency of malforma- 

 tions produced by this transitory deficiency varied with the time of instituting the 

 deficiency and with its duration and severity, providing an opportunity to study the 

 requirements for folic acid at different stages of embryogenesis. In mice, folic acid 

 analogs have produced malformations in the offspring of treated, pregnant females 

 without the necessity of using a deficient diet. 1327, 1332 



A further refinement to this approach to the analysis of the biochemistry of 

 morphogenesis made use of the nicotinamide antagonist 6-aminonicotinamide (here- 

 after called 6-AN) which is teratogenic in mice even when a corrective dose of nicotina- 

 mide is given as little as two hours after the analog is injected. 1008 The compound 

 forms a diphosphopyridine nucleotide (DPN) analog that is inactive in some DPN- 

 dependent enzymatic reactions, 227 and it is reasonable to suppose that this is the basis of 

 its teratogenicity. By varying the dose of 6-AN and the dose of the nicotinamide 

 supplement, it should be possible to obtain useful information about the relative 

 requirements of various organogenetic processes for nicotinamide. For instance, when 

 a standard dose of 6-AN is given 9.5 days after insemination and a standard dose of 

 nicotinamide is given simultaneously, no malformations occur in the offspring. When 

 the same dose of nicotinamide is given two hours after the analog many of the offspring 

 have cleft lip. When twice the amount of nicotinamide is given, again two hours after 

 the analog, the frequency of cleft lip is reduced. This suggests that a single dose of 

 nicotinamide is not sufficient to correct the metabolic block produced by the analog; 



