The relationship between the fatty acid composition of the lipids of the yolk and the brain of the duck embryo.

Abstract

Domesticated animals are often reared on formulated diets which differ greatly from the diets of their wild counterparts. In many cases. the formulated diets may be relatively deficient in n-3 polyunsaturated fatty acids, especially in docosahexaenoic acid (22:6). Since this particular fatty acid plays an important role in the embryonidneonatal development of the brain and retina, such deficiencies may result in harmfull consequences. Recent studies in this area from our laboratory have shown that the yolk lipids from eggs of alligators and ostriches currently reared in captivity are markedly deficient in n-3 fatty acids in comparison with the situation in the wild(l,2]. Moreover, this n-3 deficiency is associated with greatly increased rates of embryonic mortality. In the present study. the fatty acid composition of the yolk phospholipids from commercially-reared and free-range ducks were compared. The main difference between the two types was that the proportion of 22:6 (%w/w of total fatty acids) was approx. 3-fold greater in the freerange eggs (P<0.001). The proportion of 22:6 in the commercial duck eggs was very low; in fact yolk lipids from commercially-reared chickens typically contain considerably more 22:6[3]. However, the commercial duck eggs do contain high levels of arachidonic acid (20:4), possibly reflecting an imbalance in the proportions of n-6 to n-3 fatty acids in the formulated diets. Although the free-range ducks were free to select their diet from their environment, they were also supplied with a grain -based feed supplement. Thus it is possible that truly wild-living ducks would exhibit even greater proportions of n-3 fatty acids in their egg yolks. The consequences of the differences in the yolk 22:6 levels on the tissue lipid fatty acid profiles in the embryo were investigated. In general, the fatty acid profiles of the embryonic liver, adipose tissue and heart reflected the yolk compositions; thus the proportions of 22:6 in these tissues from the free-range embryos were approx. 4-fold greater than in the commercial embryos. The fatty acid compositions of the brain phospholipids at day 25 of the 28-day embryonic period are shown in Table I . The proportion of 22:6 in the brains of the free-range embryos was approx. 1.5-fold greater than for the commercial embryos ( P 4 . 0 0 I ) . Thus, although the differences in yolk 22:6 content in the two situations does affect the brain composition, the effects are much less severe in the brain than in the other tissues. Similar rcsults have been obtained for the alligator where a 2-fold difference in the 22:6 level in the yolks of wild versus captive animals was reflected in a 2-fold difference in the levels of this fatty acid in the lipids of the liver, adipose tissue and heart of the embryos, but with no difference observed for the brains[4] Because of the important functions of 22:6 in the lipids of the neural membranes, it is possible that the developing brain strivcs to maintain an appropriate level of this fatty acid, consistent with optimal function. Thus the fatty acid profile of the embryonic brain may be less sensitive to variations in the supply from the yolk compared with other tissues.