Marsupial and eutherian placentation

Placenta of the tammar wallaby showing the bilaminar (BOM)
and trilaminar (TOM) omphalopleure. From Guernsey et al.
eLife 2017 CC The Authors

A brand new paper compares the transcriptomes of marsupial (tammar) and eutherian (mouse and human) placentas and mammary glands (here). It confirms that marsupials have fully functional placentas expressing many of the same genes as eutherian ones.

There is evidence for a division of function between the two parts of the yolk sac placenta, with the nonvascular part (BOM) being responsible for uptake and metabolism of nutrients and the vascular part (TOM) for respiration. I am not sure how much oxygen the tiny marsupial embryo needs. Perhaps the TOM is more important for removing CO₂ and regulating the acid-base balance of the embryo. The tammar has an embryonic-type hemoglobin more capable of sequestering oxygen (protecting the embryo from reactive oxygen species) than transporting it to tissues.

A fascinating detail is that the yolk sac endoderm of the tammar has assumed functions, especially to do with trafficking of nutrients, that in eutherians are served by trophoblast.

Genes expressed in mammary gland and placenta
of marsupials and eutherians. From Guernsey et al.
eLife 2017 CC The Authors

Because much of development in the wallaby is supported by lactation, it is interesting to find considerable overlap in the transcriptomes of marsupial mammary gland and eutherian placenta.

My only criticism of this paper would be: the mouse has a yolk sac that supports early embryonic development and continues to function alongside the placenta right up to term. Perhaps the authors could not identify a data set on mouse yolk sac transcriptome, but they should have referenced the eutherian yolk sac in their discussion. An interesting theory by Claudia Freyer et al. (here) is that the stem species of therians (marsupials and eutherians) had both types of placentation.

For additional remarks on this paper see Nature News and Comments (here).

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.

The mouse yolk sac

Placenta Volume 36 Issue 10 October 2015

Great to see yolk sacs on the front cover of Placenta. Too often the mouse yolk sac is discarded or ignored in the laboratory - either through ignorance or because it has no obvious equivalent in human pregnancy. This particular study showed how mouse deficient in the OGA gene have defective blood vessel formation (vasculogenesis). This affected branching of the blood vessels in the yolk sac.

Ignorance of the yolk sac is evidenced by a high impact journal publishing a schematic where mouse yolk sac is shown to be only residual as occurs in humans (here).

I believe the mouse model of placentation is incomplete unless the yolk sac is studied along with the chorioallantoic placenta (argued here).