Embryos from C57BL/6J mice between the gestational ages of 9 and 16 days possess three embryonic haemoglobins EI, EII and EIII, the proportions of which change as a function of gestational age. Component EI, originally present at approx. 65% at day 9, decreases to approx. 20% by day 16, while component EII increases in an inverse manner to that of component EI. During this period component EIII remains essentially constant at approx. 25%. Separation of these species by ion-exchange chromatography has allowed the characterization of the Hill coefficient, Bohr effect, heat of oxygenation and binding of allosterically active organic phosphates for each component. The three components show marked functional heterogeneity and also differ from maternal haemoglobin. Oxygenation curves for whole embryonic blood show distinct deviations from simple binding behaviour. The presence of a high-affinity component within the blood samples may be accounted for by the presence of haemoglobin EI. By using parameters obtained from the study of the isolated components it has been possible to synthesize mathematically the O2-binding curves, obtained experimentally, throughout the gestational period. The characteristics of the isolated haemoglobin components of embryonic mouse blood are discussed in terms of the changing demands for O2 likely to be encountered by the developing embryo.
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