Preimplantation mouse embryos, cultured in vitro and
those freshly flushed from the reproductive tract, exhibit a switch in energy
substrate preference, from pyruvate during the early preimplantation stages,
to glucose at the blastocyst stage. Although the biochemical basis of this
phenomenon is quite well characterized, its timing and possible association
with developmental factors have not been considered. We have therefore
examined the role of five developmental factors in determining the timing of
the switch, namely: (1) embryo age (in hours post hCG);
(2) developmental stage; (3)
cytokinesis; (4) cell number; and
(5) activation of the embryonic genome. One-cell
embryos, which develop more slowly than 2-cell embryos
in vitro, were used to investigate the role of embryo
age and developmental stage. Cytochalasin D, which inhibits cytokinesis and
delays the timing of compaction and cavitation, was used to investigate the
role of cell division and developmental stage. Finally, transcription of the
embryonic genome was examined with the inhibitor, α-amanitin. Pyruvate
and glucose consumption by single embryos were measured using a non-invasive
ultramicrofluorometric technique. The results showed that the timing of the
switch in energy substrate preference is precisely regulated in the mouse
preimplantation embryo. Activation of the embryonic genome is a prerequisite
for the switch and its timing is closely associated with developmental stage,
specifically compaction and/or cavitation. Cell number, cytokinesis and
embryo age appeared to be unrelated to the timing of the switch. These
conclusions may well be extrapolated to other species, since an increase in
net glucose uptake, if not always at the expense of pyruvate, is a feature of
preimplantation embryo metabolism in all mammals studied.
Glycine supplementation in vitro enhances porcine preimplantation embryo cell number and decreases apoptosis but does not lead to live births
