Addition of interleukin-6 to mouse embryo culture increases blastocyst cell number and influences the inner cell mass to trophectoderm ratio

Objective: In vitro culture of preimplantation embryos is improved by grouping embryos together in a drop of media. Individually cultured embryos are deprived of paracrine factors; with this in mind, we investigated whether the addition of a single embryo-secreted factor, interleukin-6 (IL-6), could improve the development of individually cultured embryos. Methods: Mouse embryos were cultured individually in $2{\mu}L$ of G1/G2 media in 5% oxygen and supplemented with a range of doses of recombinant mouse or human IL-6. Results: Mouse IL-6 increased hatching at doses of 0.01 and 10 ng/mL compared to the control (93% and 93% vs. 78%, p< 0.05) and increased the total number of cells at a dose of 0.1 ng/mL compared to the control ($101.95{\pm}3.36$ vs. $91.31{\pm}3.33$, p< 0.05). In contrast, the highest dose of 100 ng/mL reduced the total number of cells ($79.86{\pm}3.29$, p< 0.05). Supplementation with human IL-6 had a different effect, with no change in hatching or total cell numbers, but an increase in the percentage of inner cell mass per embryo at doses of 0.1, 1, and 100 ng/mL compared to the control ($22.9%{\pm}1.1%$, $23.3%{\pm}1.1%$, and $23.1%{\pm}1.1%$ vs. $19.5%{\pm}1.0%$, p< 0.05). Conclusion: These data show that IL-6 improved mouse embryo development when cultured individually in complex media; however, an excess of IL-6 may be detrimental. Additionally, these data indicate that there is some cross-species benefit of human IL-6 for mouse embryos, but possibly through a different mechanism than for mouse IL-6.