Cell volume regulation in oocytes and early embryos: connecting physiology to successful culture media.
BACKGROUND
Preimplantation embryos are particularly susceptible to in vitro developmental blocks. These could be alleviated by lowering culture medium osmolarity. Because mammalian cells regulate their volumes by adjusting intracellular osmotic pressure, cell volume regulation could be critical to early embryos.


METHODS
We reviewed the literature on cell volume regulation in preimplantation embryos and the effects of increased osmolarity on embryo development, focusing also on the relation with improvements in embryo culture media.


RESULTS
Embryos failed to develop from fertilized oocytes when osmolarity is increased. This could be alleviated by decreasing osmolarity or including certain compounds such as certain amino acids. Early preimplantation mouse embryos require intracellular accumulation of glycine to provide osmotic support and thus control cell volume. The glycine-specific transporter, GLYT1, mediates osmoregulated glycine accumulation in mouse embryos and likely in human embryos. GLYT1 is activated during meiotic maturation starting at ovulation. Prior to this, oocyte size is not independently controlled but instead is determined by strong adhesion between the oocyte plasma membrane and the inner surface of the zona pellucida.


CONCLUSIONS
Early preimplantation embryos are particularly sensitive to increased osmolarity, and require the importation of glycine to regulate their cell volumes using a mechanism unique to early embryos. Cell volume regulation first appears when ovulation is triggered, oocyte zona pellucida adhesion is released, and glycine transport is activated. The requirement for supporting these physiological functions in oocytes and embryos should be taken into account when developing and improving systems for in vitro oocyte maturation and embryo culture.
Baltz
Jay M
JM
Ottawa Hospital Research Institute, Department of Obstetrics and Gynecology (Division of Reproductive Medicine), University of Ottawa, Ottawa, ON, Canada. jmbaltz@ohri.ca
Tartia
Alina P
AP
D000818:Animals; D001755:Blastocyst / Q000166:cytology* / Q000502:physiology*; D048429:Cell Size; D003470:Culture Media / Q000737:chemistry; D047108:Embryonic Development; D005260:Female; D005998:Glycine / Q000378:metabolism; D050487:Glycine Plasma Membrane Transport Proteins / Q000378:metabolism; D006706:Homeostasis; D006801:Humans; D051379:Mice; D008954:Models, Biological; D009865:Oocytes / Q000166:cytology* / Q000254:growth & development / Q000502:physiology*; D009994:Osmolar Concentration; D011247:Pregnancy
D003470:Culture Media; D050487:Glycine Plasma Membrane Transport Proteins; C493267:Slc6a9 protein, mouse; D005998:Glycine