The Qa-2 antigen, the product of the Fed (Preimplantation embryo development) gene, is a glycosylphosphatidylinositol-linked cell surface protein encoded in the Q region of the mouse major histocompatibility complex (MHC). Fed fast (Qa-2+) mouse strains have significantly higher preimplantation embryo cleavage rates both in vivo and in vitro than Fed slow (Qa-2-) mice. In this study, we determined whether the Fed fast phenotype of blastocysts is due to an increased number of blastomeres in the trophectoderm (TE), the inner cell mass (ICM), or both. We also analysed the Fed gene expression pattern, both at the mRNA and at the protein level, in these lineages. Blastocysts were collected from the congenic mouse strains B6.K2 (Qa-2+) and B6.K1 (Qa-2-). We performed reverse transcription-polymerase chain reaction (PCR) and Immuno-PCR and found that the Fed gene is expressed at the mRNA and protein level in whole embryos and in isolated ICM cells. Lastly, we differentially stained embryos from these strains and found that B6.K2 blastocysts had significantly higher cell numbers (P < 0.05) in both the ICM and in the TE than B6.K1 blastocysts. These results suggest that Qa-2 expression in both the TE and the ICM of blastocysts directly contributes to the Fed phenotype.
