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Avian embryo development is associated with a progressive accumulation in certain tissues of lipids which are rich in polyunsaturated fatty acids (PUFA), making such tissues vulnerable to free radical attack and lipid peroxidation. Antioxidant systems are expressed in the embryonic tissues and are responsible for maintaining antioxidant protection during different stages of embryoonic development, in particular at hatching time when oxidative stress is highly pronounced (Surai, 1999). The antioxidant systems of the avian embryo are based on the antioxidant enzymes superoxide dismutase (SOD), glutathione peroxidase (GSH-Px) and catalase (Surai, 1999a ; Surai, 2000a), and fat-soluble and water-soluble natural antioxidants (Surai, 1999). Vitamin E is considered to be the main player in the antioxidant defence at the cellular and subcellular levels (Niki,1996). In recent years it has become evident that in the cell the association of vitamin E with other natural antioxidants which are able to recycle this vitamin from its oxidized form is the major biochemical mechanism in maintaining the efficiency of antioxidant defence (Surai, 1999; Surai, 2000). Different antioxidant components occupy distinct cellular and extracellular compartments and among them, active recycling plays a crucial role. For example, oxidized vitamin E (tocopheroxyl radical) has been shown to be regenerated by ascorbate, glutathione (GSH), lipoic acid, or ubiquinols. Furthermore glutathione disulfides (GSSG) can be regenerated by glutathione reductase and enzymic pathways have also been identified for the recycling of the ascorbate radical and dehydroascorbate. The reducing agents that are used to fuel these recycling reactions (NADH and NADPH) are ultimately derived from the oxidation of nutrients (Chap et al., 1999). The exact role of carotenoids within this system is still under discussion but evidence is accumulating concerning antioxidant properties of carotenoids and their role in antioxidant defence directly or via interactions with other antioxidants increasing their efficiency. In this respect, development of effective HPLC-based methods for carotenoid analysis, in association with the introduction of new assays for antioxidant activities in vitro, promise new advances in this area.

Carotenoids in Avian Nutrition and Embryonic Development. 1. Absorption, Availability and Levels in Plasma and Egg Yolk