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.

Is the Embryo Like an Acorn Rather than an Oak Tree?

Michael Sandel is a professor of political theory at Harvard University and was a member of President George W. Bush’s President’s Council on Bioethics.

Sandel states that even though every oak tree was at one time in its life an acorn, “it does not follow that acorns are oak trees.” Why? Compare how we might regard the loss of a magnificent oak tree over the loss of a single acorn. While the loss of the oak tree is received with distress and disappointment, the loss of the acorn hardly evokes a response. The same kind of disparate response is evoked by the loss of a mature human being over the loss of a human embryo. Therefore just as an acorn is a different kind of thing from an oak tree, a human embryo is a different kind of thing than a mature human person.

This argument has a lot of things wrong with it. In the first place, it begins with a significant biological error. Acorns and oak trees are both oaks, that is, members of the genus Quercus. They are the same organism at different stages of development. Sandel acknowledges this at first, and then denies it based on features that the oak tree acquires later in its development.

Secondly. Robert George and Franciscan University of Stubenville philosopher Patrick Lee have pointed out that the characteristics Sandel uses to categorize oak trees and acorns into different groups, like grandeur, beauty and so on, are “accidental” attributes. This is just a fancy way of describing a feature that is incidental to who you are and not essential. You can change an accidental attribute of something without affecting what it is. A good example is skin color in humans. People can have red, yellow, black or white skin and still be people. If their skin color changes, like when they get a tan in the summer and lighten up in the winter, they are still people. Herein lies the whole reason why it was evil for people with lighter skin to enslave those who had darker skin. Even though some argued that people with dark skin were not human persons, this is plainly ridiculous because a person can have light or dark skin and still be a person – skin color is an accidental attribute. Thus, a scrawny, scruffy, diseased oak tree with twisted bark is just as much an oak tree as a large, grand one. The grandeur, size, and beauty of the tree are accidental features that do not make it an oak tree. Sandel has made accidental features of oak trees and humans the most important things about them even though they are not.

George and Lee make other telling points against Sandel’s analogy. If acorns are to embryos as oak trees are to people, then what about oak saplings? Forest managers often cull oak saplings to prevent excessive crowding of trees and promote the health of the forest, and no one has any misgivings about such a practice. Yet if acorns are like human embryos then oak saplings are like human toddlers. We would not entertain culling toddlers. We would also not have any trouble with pulling up and burning a diseased, disheveled oak tree, but killing the mentally or physically disabled would be just plain incorrigibly evil. Clearly, Sandel’s analogy simply does not work.

Finally, Sandel’s main point seems to pivot around how he feels about the embryo. According to Sandel an oak tree and acorn are not the same kind of thing because we feel much more strongly attached to an oak tree than to an acorn. This is revealed in Sandel’s statements that “more than half of all fertilized eggs either fail to implant or are otherwise lost,” and “the way we respond to the natural loss of embryos suggests that we do not regard this event as the moral or religious equivalent of the death of infants.” Sandel gets his facts wrong about the percentage of embryos that die, but his assertion that the lack of a funeral means that the embryo is not a human person is a troubling one. What is it about having a funeral after you die that makes you a person? Did people slaughtered by Saddam Hussein, Pol Pot, or Adolph Hitler and buried in mass graves have a funeral? Were they not human persons? Of course they were. The embryo normally dies before we know anything about it or have a chance to become attached to it. Just because we do not hold a funeral for its demise is neither here nor there.

I think Sandel’s ethic gets even more sinister if we take it to its logical conclusions. My daughter just returned from a mission trip to a homeless shelter in Minneapolis, Minnesota. While there, she and her friends encountered a homeless lady who was afflicted with bipolar disorder. Her parents lived four doors down from her daily haunt, but never came to even talk to her or invite her home. Previously, she was even in a coma for several weeks, but her parents never came to see her or gave the slightest indication that they cared about her condition. This homeless lady’s parents have completely abandoned their own daughter. They don’t like her mental illness. In short, they don’t “feel” like being her parents anymore. Under Sandel’s ethical criteria, what these parents did was morally fine. Yet any parent worth their moral salt will tell you that their children are ALWAYS their children, and this homeless lady’s parents have failed in their most basic duty to their own daughter. Such neglect is an outrage, but if we take Sandel seriously, this lady’s parents were morally upright in all they did. The consequences of Sandel’s treatment are heinous, unchristian and unworkable for any society.

In the end Sandel has offered a troubling recipe for justifying the destruction of embryos – because he does not feel a strong attachment toward them. Such thinking is not only unconvincing, but morally dangerous.