Metabolism of the viable mammalian embryo: quietness revisited

Abstract

This review examines the 'Quiet Embryo Hypothesis' which proposes that viable preimplantation embryos operate at metabolite or nutrient turnover rates distributed within lower ranges than those of their less viable counterparts. The 'quieter' metabolism consistent with this hypothesis is considered in terms of (i) 'functional' quietness; the contrasting levels of intrinsic metabolic activity in different cell types as a consequence of their specialized functions, (ii) inter-individual embryo/cell differences in metabolism and (iii) loss of quietness in response to environmental stress. Data are reviewed which indicate that gametes and early embryos function in vivo at a lower temperature than core body temperature, which could encourage the expression of a quiet metabolism. We call for research to determine the optimum temperature for mammalian gamete/embryo culture. The review concludes by examining the key role of reactive oxygen species, which can induce molecular damage, trigger a cellular stress response and lead to a loss of quietness.

Figure 1:

ATP production by in vitro-derived porcine embryos calculated from lactate production and oxygen consumption. There is a characteristic shift in the metabolic profile of early porcine embryos, with an increase in the amount of ATP produced but the relative contribution from glycolysis is minimal. The methods used in generating these data are described in Sturmey and Leese (2003).