Single-cell proteomics of pre-implantation mouse embryos uncovers distinct asymmetry of certain proteins among early blastomeres

During pre-implantation mouse embryonic development, blastomeres undergo division and differentiation, reaching a distinctive level of heterogeneity, hence the completion of the first cell fate determination. However, when the initial asymmetry emerges and how this heterogeneity amplifies, particularly at the protein level, remain elusive. Here, by mass spectrometry-based single-blastomere proteomics, we identified proteins exhibiting significant heterogeneity in abundance among mouse blastomeres as early as the 2-cell stage. Differential gene expression among blastomeres, as indicated by intra-embryo variation in RNA abundance detected through single-cell RNA sequencing, was insufficient to fully explain the corresponding disparities in protein abundance. Instead, the asymmetric distribution of protein molecules during cell division was observed, serving as another mechanism contributing to protein heterogeneity, independent of RNA expression.