Foetal and Placental Growth in the Mouse After Pre-implantation Development in Vitro Under Oxygen Concentrations' of 5 and 20%

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Abstract
Blastocysts which developed from 2-cell mouse embyros in culture tubes containing an atmosphere with 20% O2 had .apprx. 20% fewer blastomeres than blastocysts which developed under an O2 concentration of 5%. When these smaller blastocysts were transferred to the uteri of pseudopregnant foster mothers, the fetuses developing were as viable as those developing from blastocysts cultured under 5% O2 indicating their ability to regulate for a lower blastomere number by at least day 17 of development. The transfer operation itself had no adverse effect on fetal or placental growth. Blastocyst cultures in vitro did depress fetal though not placental growth, suggesting that the inner cell mass was more susceptible than the trophectoderm to culture in vitro. Fetal but not placental growth was lower following the transfer of blastocysts to a day 3 rather than a day 4 uterus. Cases [4] of placental fusion were found. In 1 case, the fetuses were contained within the same embryonic sac and were possibly twins.