Glycine transport by single human and mouse embryos.
Mouse zygotes and early cleavage-stage embryos have previously been shown to utilize glycine as an organic osmolyte, accumulating it to oppose any decrease in cell volume. Such glycine uptake in early cleavage-stage mouse embryos is via the glycine-specific Gly transporter. Mouse embryos also possess swelling-activated channels which function to release osmotically active glycine and other osmolytes when cell volume becomes too large. In this study it was found that human cleavage-stage embryos also transported glycine via a similarly saturable, sarcosine-inhibitable transporter, implying that the Gly transporter also mediates glycine transport in human embryos. Mouse zygotes have previously been shown to accumulate more intracellular glycine when cultured at increased osmolarities for 24 h. It was found in the current study that this ability was lost as preimplantation mouse embryo development proceeded, and that early cleavage-stage human embryos may also be capable of such osmosensitive accumulation of glycine. Finally, using spare human eggs which had failed to fertilize or cleave, the presence of swelling-activated currents resembling those in mouse zygotes was demonstrated. These data indicate that osmoregulation in early human embryos occurs via similar mechanisms as in the mouse.