Uterine fluid (UF) consists of lipids, metabolites, proteins and hormones, and is secreted by the endometrial glands. While UF is essential for early embryo development and blastocyst implantation, its importance post-implantation, i.e. after placenta formation, is not well understood. Hence, it is routine to grow mouse zygotes up to the blastocyst stage in culture, but the factors required for achieving development beyond this stage remain unknown.
Lipids are essential for embryo development through their involvement in maintaining cell structure, cell signalling, neuronal development, steroid biosynthesis and are an energy source. We have characterised the lipidome of mouse UF across gestation (6 timepoints) using mass spectrometry, which identified 971 lipid species. Principal component analysis demonstrates that early stages cluster together, while mid- and late-stages cluster individually, indicating a shift in the lipidome post-implantation. Significant changes were detected in total levels of sphingolipids, glycerophospholipids, glycerolipids, and sterols in mid- and late-stages of gestation, indicating the need for different lipids at specific timepoints.
For example, glycosphingolipids such as hexosylceramides (HexCer) and dihexosylceramides (Hex2Cer) are involved in cellular processes such as cell proliferation, differentiation, migration and angiogenesis and are required for embryo survival post implantation. Accordingly, our data demonstrate that total levels as well as specific species of hexosylceramides and dihexosylceramides progressively increase in UF post-implantation with up to 23-fold higher levels in late- compared to early-gestation. Glycosphingolipids are synthesised by the glucosyltransferase enzyme (UGCG) and its expression in the uterus across gestation will determine if the endometrial glands are the source of the increased glycosphingolipids.
In summary, our data provide insight into the role of each lipid class and how lipids in the UF impact embryo development. This will allow for the identification of components that can be supplemented into ex utero culture media to prolong culture of early-, mid- and late-stage embryos.