Impact of the type of incubator (non-humidified versus humidified) on embryo culture media osmolality

Introduction. - Benchtop incubators with small individual chambers have been developed in order to improve the stability of embryo culture conditions reducing the environmental stress during the embryo development. These new dry incubators were designed without any air humidification system in order to prevent bacterial proliferation and to enable the use of time-lapse system. However, an elevated evaporation of the culture media could occur in these conditions. The main objective of the study is to analyse the impact of the used incubator type on the embryo culture media osmolality. Materials and methods. - Microdrops of 50 mu L of culture media were placed in 60 mm diameter culture dishes, and quickly covered with either 7 or 8 mL of mineral oil in an IVF workstation with laminar airflow. Two series of culture dishes have been randomly placed either in a humidified incubator or in a dry benchtop incubator. The microdrops of each culture dishes were sampled at D0, D1, D2, D3, and D5 respectively to measure the osmolality in triplicate using a cryoscopic osmometer. The mean values of osmolality in each incubator have been compared respectively on D0, D1, D2, D3 and D5 with appropriate statistical tests, and considered statistically significant when P < 0.05. Results. - The osmolality of the microdrops placed in the dry benchtop incubator differed significantly after the third day of culture, regardless of the level of mineral oil in the culture dishes. Indeed, using Petri dishes covered respectively with 7 or 8 mL of mineral oil, osmolality values of samples from the dry incubator were significantly higher than those from the humidified one, at D3 and D5 (D3/7 mL: 273 +/- 2.1 vs. 268 +/- 1.0 mOsm/kg; P = 0.02; D3/8 mL: 282 +/- 8.0 vs. 270 +/- 0.7 mOsm/kg; P = 0.04) and D5 (D5/7 mL: 283 +/- 1.5 vs. 270 +/- 3.6 mOsm/kg; P = 0.004; D5/8 mL: 287 +/- 5.6 vs. 268 +/- 2.3 mOsm/kg; P = 0.005). Furthermore, the analysis on paired samples showed that the osmolality in the dry benchtop incubator at D5 using 7 mL of oil (283 +/- 1.5 mOsm/kg; P = 0.003) and at D3 (273 +/- 2.1 mOsm/kg; P = 0.016) and D5 (287 +/- 5.6 mOsm/kg; P = 0.009) using 8 mL of oil was significantly higher than that measured at D0 (265 +/- 1.9 mOsm/kg). Conclusion. - A significant increase of the embryo culture media osmolality was observed in the dry benchtop incubator with ambient hygrometry in our standard conditions. Adding 1 mL of oil was not sufficient to reduce the evaporation of the media. Although maintained at a physiological level, the impact of the osmolality changes on the in vitro embryo development has to be further determined. (C) 2020 Elsevier Masson SAS. All rights reserved.