Mouse and human blastocysts compared

Mouse blastocyst with trophectoderm (TE),
epiblast (EPI) and primitive endoderm (PE)  from Selenka 1883

Even before implantation, three cell lineages are apparent in the blastocyst of mouse and human. Outermost is the trophectoderm that will contribute trophoblast to the placenta. The inner cell mass has already differentiated into the epiblast and the primitive endoderm or hypoblast. For mouse, this much has been clear since the pioneering studies of Emil Selenka (here).

Gene expression in these three lineages of mouse placenta has been known for some time. What does this tell us about human preimplantation development? A recent paper in Development (here) suggests less than some might like to think.

• For trophectoderm, key lineages expressed in the mouse (e.g. Id2, Elf5, Eomes) either are not expressed in human trophectoderm or are expressed in alternative lineages.
• There are several genes that are exclusively expressed in human epiblast (e.g.the transcription factor KLF17).
• Expression of some genes in primitive endoderm is conserved between mouse and human (e.g. Foxa2/FOXA2).

These findings support other work indicating that the genes and signalling pathways involved in lineage specification differ between mouse and human blastocysts.