Developmental Regression: Making Placental Cells from Embryonic Stem Cells

A research group from Copenhagen, Denmark has discovered a way to make placental cells from embryonic stem cells. In order to do this, the embryonic stem cells must be developmentally regressed so that they can become wither placenta-making cells rather than inner cell mass cells.

This study is significant for two reasons. First of all, it was thought to be impossible to make placental cells from embryonic stem cells because embryonic stem cells (ESCs) are derived from the inner cell mass cells of 4-5-day old human blastocysts. These early embryos begin as single-celled embryos that divide to form 12-16-cell embryos that undergo compaction. At this time, the cells on the outside become trophoblast cells, which will form the trophectoderm and form the placenta and the cells on the inside will form the inner cell mass, which will form the embryo proper and a few extraembryonic structures. Since ESCs are derived from inner cell mass cells that have been isolated and successfully cultured, they have already committed to a cell fate that is not placental. Therefore, to differentiate ESCs into placental cells would require that ESCs developmentally regress, which is very difficult to do in culture.

Secondly, if this could be achieved, several placental abnormalities could be more easily investigated, For example, pre-eclampsia is a very serious prenatal condition that is potentially fatal to the mother, and is linked to abnormalities of the placenta. Studying a condition such as pre-eclampsia in a culture system would definitely be a boon to gynecological research.

Because human ESCs can express genes that are characteristic of trophoblast cells if they are treated with a growth factor called Bone Morphogen Protein 4 (BMP4), it seems possible to make placental cells from them (see Xu R.H., Chen X., Li D.S., Li R., Addicks G.C., Glennon C., Zwaka T.P., Thomson J.A. BMP4 initiates human embryonic stem cell differentiation to trophoblast. Nat. Biotechnol. 2002;20:1261–1264, and Xu RH. Methods Mol Med. 2006;121:189-202). However, a study by Andreia S. Bernardo and others from the laboratory of Roger Pedersen at the Cambridge Stem Cell Institute strongly suggested that BMP4 treatment, even in the absence of FGF signaling (another growth factor that has to be absent for BMP4 to induce trophoblast-like gene expression from ESCs) the particular genes induced by BMP4 are not exclusive to trophoblast cells and more closely resemble mesodermal gene profiles (see AS Bernardo, et al., Cell Stem Cell. 2011 Aug 5;9(2):144-55).

Into the fray of this debate comes a paper by stem cells scientists at the Danish Stem Cell Center at the University of Copenhagen that shows that it is possible to rewind the developmental state of ESCs.

In this paper, Josh Brickman and his team discovered that if they maintained mouse ESCs under specific conditions, they could cause the cells to regress into very early pre-blastocyst embryonic cells that can form trophoblast cells or ICM cells.

“It was a very exciting moment when we tested the theory, said Brinkman. “We found that not only can we make adult cells but also placenta, in fact we got precursors of placenta, yolk sac as well as embryo from just one cell.”

“This new discovery is crucial for the basic understanding of the nature of embryonic stem cells and could provide a way to model the development of the organism as a whole, rather than just the embryonic portion,” said Sophie Morgani, graduate student and first author of this paper. “In this way we may gain greater insight into conditions where extraembryonic development is impaired, as in the case of miscarriages.”

To de-differentiate the ESCs, Brinkman and his colleagues grew them in a solution called “2i.”  This 2i culture medium contained inhibitors of MEK and GSK3.  MEK is a protein kinase that is a central participant in the “MAP kinase signaling pathway, which is a signaling pathway that is central to cell growth and survival.  This particular signaling pathway is the target of the anthrax toxin, which illustrates its importance,  GSK3 stands for “glycogen synthase kinase 3,” which is a signaling protein in the Wnt pathway.

When the mouse ESCs were grown in 2i medium they expressed genes normally found only in pre-blastocyst embryos (Hex, for example).  Therefore, the 2i medium directs mouse ESCs to de-differentiate.  When ESCs grown in 2i were implanted into mouse embryos, they divided and differentiated into cells that were found in placental and embryonic fates.  This strongly argues that the ESCs grown in 2i became pre-blastocyst embryonic cells.  When the ESCs grown in 2i were also grown with LIF, which stands for “leukemia inhibitory factor” (LIF is a protein required for the maintenance of mouse ESCs in culture), the 2i cells were maintained in culture and grew while maintaining their pre-blastocyst status.  These cells differentiated into placental cells, embryonic or fetal cells.  Essentially, the 2i-cultured cells when from being pluripotent to being “totipotent,” or able to form ALL cell types in the embryo, fetus, or the adult.

ESC de-differentiation in totipotence

“In our study we have been able to see the full picture unifying LIF’s functions: what LIF really does, is to support the very early embryo state, where the cells can make both embryonic cells and placenta. This fits with LIFs’ role in supporting implantation,” said Brinkman.

This study definitively shows that ESCs are NOT embryos.  ESCs can regress in their development but embryos develop forward, becoming more committed as they develop and more restricted in the cell fates they can form.  This should effectively put the nail in the coffin of Lee Silver’s argument against Robert P. George that embryonic stem cells are embryos.  They are definitely and unequivocally, since embryos do NOT develop in reverse, but ESCs can and do.

Robert P. George argues that early human embryos, like the kind used to make ESCs are very young  members of the human race and deserve, at the minimum, the right not to be harmed.  Silver counters that George’s argument is inconsistent because George would not extend the same right to an ESC cell line, which is the same as an embryo.  His reasoning is that mouse ESCs can be transplanted into other mouse embryos that have four copies of each chromosome.  The messed up mouse embryo will make the placenta and the ESCs will make the inner cell mass and the mouse will develop and even come to term.  This is called tetraploid rescue, and Silver thinks that this procedure is a minor manipulation, but that it shows that ESCs are functionally the same as embryos.

I find Silver’s argument wanting on just about all fronts.  This is not a minor manipulation.  The tetraploid embryo is bound for certain death, but the implanted ESCs use the developmental context of the tetraploid embryo to find their place in it and make the inner cell mass.  The ESCs do not do it all on their own, but instead work with the tetraploid embryo in a complex developmental give-and-take to make an embryo with the placenta from one animal and the embryo proper from another.

Thus Silver’s first argument does not demonstrate what he says it does.  All it demonstrates is that ESCs can contribute to an embryo, which is something we already knew and expected.  This new data completes blows Silver’s assertion out of the water, since ESCs can take developmental steps backward and embryos by their very nature and programming, do not.  Thus these two entities are distinct entities and are not identical.  The early embryo is a very young human person, full stop.  We should stop dismembering them in laboratories just to stem our scientific curiosity.