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).