Comparative composition and utilisation of yolk lipid by embryonic birds and reptiles

The presence of an extensive yolk mass to supply the developing embryo with a large proportion of its nutrients is a feature common to all birds and reptiles. The yolk size, the proportional distribution of the major nutrients within the yolk and the rates of utilisation of the components during embryonic development differ widely between species. However, the major feature of all yolks is a high initial lipid content and rapid lipid utilisation during the later stages of embryo development when growth is maximal (Romanoff, 1960; Noble & Moore, 1964; Manolis, Webb & Dempsey, 1987). The yolk lipid performs a role both as the major energy source and as a supply of nutritionally essential tissue components (Romanoff, 1960). Utilisation of yolk lipid, therefore, constitutes a major part of the interrelated chain of events required for successful hatching and is characterised by extensive and, in many instances, distinctive metabolic features that are quite unique (Noble, 1987a; Noble et al., 1990a). Much is known concerning the utilisation of the yolk lipid during embryonic development in birds, especially the fowl (Gallus gallus) but similar precise and extensive data for lipids and lipid utilisation in reptiles are largely unavailable. However, the importance of understanding yolk lipid uptake in reptiles is of increasing importance, especially in view of the increasing need for captive breeding for conservation and commercial purposes (Ferguson, 1985).