The utilization of yolk lipids by the chick embryo

The lipids of the yolk represent the primary nutrient source for the chick embryo, providing over 90% of the energy required for development as well a s supplying a range of structural components for membrane biogenesis. Since the energy needs and the requirements for particular fatty acids differ gr eatly among the various embryonic tissues and between successive developmen tal stages, the regulated distribution of lipid components from the yolk to the different body sites forms a central feature of this system. As a cons equence, the various tissues of the newly hatched chick display a range of highly characteristic lipid and fatty acid compositions in accordance with their functions but which, in many cases, also reflect the intensity and co mplexity of the lipid transfer process in the embryo. For example, the live r of the chick contains very high levels of cholesteryl ester as a result o f the mechanism of lipid transfer. Moreover, the triacylglycerol fraction o f the liver is highly enriched in docosahexaenoic acid, possibly reflecting hepatic involvement in the transfer of this fatty acid to the brain. The a ccretion of large amounts of both docosahexaenoic and arachidonic acids in the phospholipids of the brain and retina is a key feature of chick embryo development, the importance of which is underlined by the expression of a s eries of mechanisms, possibly mediated by the liver and adipose tissue, des igned to promote the transfer of these fatty acids to the neural tissues.