CAROTENOIDS 



xanthophylls ' of the yolk do not appear to be used up in any way 

 At hatching the liver of the chicks contains about 8 per cent, of the 

 total body carotenoids. ' The liver and plasma carotenoid levels are 

 high at hatching but rapidly decrease during the first week of life. ^ * 

 Wald and Zussman ^ o investigated the eyes of developing embryos ; 

 pink droplets (containing astaxanthin) appeared on the 19th day of 

 incubation and the yellow droplets (sarcinene) somewhat later. This 

 shows that these pigments must either be totally synthesized in the 

 retina, or formed by oxidation of the yolk carotenoids in a unknown 

 site and are then transported to the retina. 



The second alternative is much more probable, for such a process 

 has been demonstrated in the production of plumage carotenoids. 

 Brockmann and Volker^' produced white birds when canaries were 

 reared on a carotenoid-free diet. Only xanthophylls were precursors 

 of the canaryxanthophyll, for pigmentation of the feathers only occurred 

 when lutein (xanthophyll) or zeaxanthin, but not carotene or lycopene, 

 were included in the diet. Violaxanthin did not produce pigmentation 

 but its failure was ascribed to its instability in the gastric juice of 

 canaries, although it may not have been absorbed (see p. 259). 



In bishop birds, Kritzler 3 ?, 3 s found that lycopene was a precursor 

 of pigment R2 ; captive birds produced only small amounts of R2 

 unless fed tomatoes. Capsanthin was carried to the feathers unchanged. 



FUNCTION 



It has been noted that carotene functions in birds by acting as a 

 vitamin A precursor, and it is converted into vitamin A with consider- 

 able efficiency. The biological functions of the xanthophylls which 

 birds so assiduously store are, on the other hand, still obscure. 



Palmer and Kempster * * considered that in hens at least xanthophylls 

 are of no physiological importance and that mobilization from the 

 shanks to the eggs is merely due to the fact that eggs are a convenient 

 excretory route for fat-soluble substances. They also managed to rear 

 perfectly normal chickens with normal fecundity and fertility on 

 xanthophyll-free diets. Recent work by Schumacher, Scott, Hughes 

 and Peterson ' ^ and by Bohren, Carrick and Andrews ' * confirms this. 



Other workers, whilst not denying all biological function to xantho- 

 phylls are, with one exception, convinced that they have no vitamin A 

 activity in the chick. ' 5- s Euler and Klussman, ' ^^ however, believed, 

 in 1931, that lutein (xanthophyll) was indeed a vitamin A precursor in 

 the chicken. It has been suggested that lutein (xanthophyll) is 

 converted into an essential growth factor differing from vitamin A but 



266 



