Imaging lipids in living mammalian oocytes and early embryos by coherent Raman scattering microscopy

Many promising techniques proposed to monitor gamete developmental potential and quality are invasive and not realistically useful in clinical practise. Hence, there is increasing interest in the development of non-invasive imaging methods that can be applied to mammalian eggs and early embryos. Recent studies have shown that mammalian oocyte and embryo viability are closely associated with their metabolic profile, relying entirely on mitochondria as a source of ATP. Fatty acids, stored in intracellular lipid droplets, are an important source of ATP. We have recently demonstrated the use of Coherent Anti-stokes Raman Scattering (CARS) microscopy to allow chemically-specific, label-free imaging of lipid droplets, with high three-dimensional spatial resolution. Here, we summarize our main findings when using CARS to examine the number, size, and 3D spatial distribution of lipid droplets in mouse eggs and early embryos. Quantitative analysis showed statistically significant differences during oocyte maturation and early embryo development. Notably, CARS imaging did not compromise maturation or development. In mouse oocytes that had been subjected to alterations in mitochondrial metabolism we found that the spatial distribution pattern of lipid droplets was also altered. In addition, differences in the chemical composition of lipid droplets in living oocytes matured in media supplemented with different saturated and unsaturated fatty acids were detected using CARS hyperspectral imaging. We also imaged bovine oocytes, and found that lipid droplets appear to be larger and with less spatial aggregation than in mouse oocytes, possibly reflecting the fact that different species metabolise lipids differently. These data suggest that CARS microscopy is a promising non-invasive technique for assessing specific aspects of the metabolic profile of living mammalian eggs and early embryos, which could be potentially linked to their quality and viability.