Mineral oil viscosity and quantity impact osmotic pressure, not embryo development

Dry incubators prevent bacterial growth and allow time-lapse imaging. However, they cause more significant osmotic pressure changes in the culture medium than in humidified incubators. Few reports have explored the effect of osmotic pressure on human embryos cultured under different conditions. This study examined how changes in osmotic pressure affect human embryos in a dry incubator. The study incubated embryos in culture mediums covered with mineral oil of varying viscosities and quantities. The osmotic pressure of the culture medium was measured daily for six-day period (Day 0-6) in four experimental groups established by varying the viscosity and volume of mineral oil: low viscosity (Light), 3.0 ml or 4.5 ml, and high viscosity (Heavy), 3.0 ml or 4.5 ml of mineral oil. The Light 3.0 ml and Heavy 4.5 ml groups, showing the greatest difference in the osmotic pressure, were used to culture of human embryos. After six days of incubation, the osmotic pressure increased the most in Light 3.0 ml group. Heavy 4.5 ml group had the smallest change. However, no significant differences were noted in the formation rates of blastocysts, good-quality blastocysts, or cell count between the two groups. The study suggests that even when the culture medium is covered with heavy mineral oil in a dry incubator, osmotic pressure increases after six days but does not significantly affect the formation of blastocysts. These findings provide valuable insights into the effects of varying osmotic pressure on embryonic development and may help in optimizing conditions for <i>in vitro</i> fertilization.