Soluble HLA-G and embryo implantation: frequently asked questions

Two major criteria are currently used for optimal human embryo selection in in vitro fertilization or intracytoplasmic sperm injection: overall morphology-based grading system and rate of fragmentation. The measurement of soluble HLA-G (sHLA-G) in human preimplantation embryo culture supernatants gives great hope that addition of such a biochemical quantitative criterion would further improve the rates of implantation. In this short review, we will examine the functional significance of this latter approach by discussing the following frequently asked questions: (1) Which sHLA-G isoform(s) might be present in the human preimplantation embryo culture supernatants? (2) Where do these sHLA-G molecules come from? (3) Why are some preimplantation embryos 'HLA-G secretors' and others not? (4) How might the functions exerted by sHLA-G molecules influence embryonic implantation?