Table I.

Proposed categories of metabolic ‘Quietness’.

Category of ‘Quietness’	Examples
Functional Quietness	Cleavage stage embryos are quieter than blastocysts
	Cells of the inner cell mass are quieter than trophectoderm
	Metabolic rate is dramatically reduced in embryonic diapause
Inter-individual embryo/cell differences in quietness	Quiet embryos are more viable than active
	Do quiet ICM cells go on to form embryonic stem cells in culture?
	Are quieter cells/blastomeres less likely to apoptose?
Loss of Quietness, e.g. in response to environmental stress	In vivo-derived embryos are metabolically less active than in vitro-produced
	Exposure to the environment in vitro elevates metabolic activity of in vivo embryos
	High plane of maternal nutrition/feeding leads to increased embryonic metabolism
	Serum and ammonium in culture medium increase metabolic activity
	Increased plasma ammonium levels (in response to diet) increase embryo metabolism
	Accelerated, precocious embryo development up-regulates metabolic activity
	Exposure to elevated oxygen may increase ROS production
	Gamete development occurs at a reduced body temperature