The Transfer of Essential Polyunsaturated Fatty Acids From the Yolk to the Tissues During Avian Embryonic Development

The quality of the nutrients provided to the developing embryo are of primary importance to ensure the proper functional development of the various organs. The brain and retina require large amounts of certain long chain polyunsaturated fatty acids (PUFA) during embryonic and neonatal life. Docosahexaenoic acid (DHA) is a major component of neural tissues. However, the mechanism of delivery of this fatty acid to the brain still has to be established. The chick embryo was used as model system in the present study. The amounts and proportions of the major fatty acids in the lipids of the yolk, whole embryo, blood plasma, adipose tissue, heart and brain were determined throughout chick embryo development. The results revealed a series of unique features associated with various stages of DHA transport from the yolk to the target tissues. Firstly, DHA was preferentially transferred from the lipids of the yolk complex to the embryo. Secondly, since DHA was present in the initial yolk as a component of phospholipid, but was released into the embryonic circulation in the form of plasma triacylglycerol; it was proposed that DHA is translocated from phospholipid to triacylglycerol in the yolk sac membrane. It was also proposed that the high activity of lipoprotein lipase in the developing adipose tissue promotes the hydrolysis of DHA from plasma triacylglyceroi and the incorporation of this DHA into the triacylglycerol of the adipocytes. The pattern of change in the amount of DHA present in adipose triacylglycerol suggested that this fatty acid may be preferentially mobilised from this tissue to be released into the circulation as the free fatty acid. Experiments involving the incubation of adipose tissue pieces in vitro also provided evidence for the preferential mobilisation of DHA from the tissue. It was proposed that DHA in the form of plasma free fatty acid is readily taken up by the brain. The amount of DHA in brain phospholipid increased continuously during embryonic development but this increase occurred in parallel with the accretion of the other major fatty acids; i.e. there was no evidence for any preferential incorporation of DHA. Brain phospholipid classes were separated using an HPLC system and it was shown that phosphatidylethanolamine and phosphatidylserine were the major DHA-containing fractions in this tissue. Similar results were also obtained using duck embryo brain. On the basis of the findings of this study, a mechanism for the transfer of DHA from the yolk to brain is proposed; the key feature of this mechanism is the role of adipose tissue as a mediator of this process. The possible relevance of these findings to mammalian and human development is discussed