How do embryos talk?

When did you start speaking? Conventional wisdom would suggest that it would be around the one-year old mark. The conventional wisdom could be off by quite a bit in this case. I believe that the first word of our lives was uttered even before we opened our eyes.

We have been talking since we were embryos. Yes, embryos do talk. The language is not well understood, and it is inaudible, but embryos do speak to their mothers even before implanting in the uterus. In fact, one big reason we are here is probably because we were such big talkers in utero.

Since the inception of in vitro fertilization in 1977, the rate of successful pregnancies has not increased beyond 50%. One of the biggest causes for the low success rate is the failure of the embryo to implant in the endometrium, the innermost lining layer of the uterus.

Scientists have been busy trying to come up with reasons for the low amount of implantation. One of the hypotheses trying to describe the phenomenon, posits that embryos and the endometrium communicate with each other and this communication, if successful, causes the endometrium to change chemically, physiologically and morphologically bringing about the “window of implantation” which leads to a successful implantation.

In the Transgeno research group lead by the University of Tartu professor Alireza Fazeli, we are very much interested in baby talk, that is embryo-maternal communication.

Since it would be highly unethical to test any actual human embryos for embryo-maternal communication, we have used a malignant cell-line based system to replicate the microenvironment immediately prior to embryo implantation.

This is an enlarged view of an embryo in the mother’s uterus. JAR cells were used to mimic the outermost cell layer of the embryo, while RL 95-2 cells were used to mimic the innermost lining layer of the uterus.
Image credit: Kasun Madhuranga Godakumara Godagedara

One cell type, JAR, was used to mimic the trophoblast, the outermost cell layer of the embryo and another cell type, RL 95-2, was used to mimic the endometrium. JAR cells were formed into a 3D structure called a spheroid to imitate a trophoblast better.

Testing the hypothesis that the messages used in the communication are in the form of RNA, we have labelled all the RNA in the trophoblast spheroids. Since we have labelled the living cells, the process is termed bio-orthogonal labelling. When we put the two cell types together, we could track the transfer of embryonic RNA from the trophoblast to the endometrium, demonstrating the validity of the hypothesis. We observed that not only some species of RNA are indeed transferred from the embryo to the endometrium, but also the endometrial RNA changes because of the transfer.

Then we looked into the method of message transfer. Naturally, we started from the extracellular vesicles – the cargo ships of the cell world. Extracellular vesicles or EVs are very small (about 100nm in diameter) packages containing lipids, proteins, and nucleic acids, that is DNA and RNA. Once considered as just cellular junk, EVs and EV research has become one of the hottest topics in the scientific world in recent years.

We isolated the EVs produced by the JAR spheroids and supplemented the endometrial cells. Interestingly, the EVs were able to produce an identical change in endometrial RNA compared to the actual JAR spheroids. EVs seemed to be a major component in the embryo-maternal communication mechanism.

An embryo is essentially a parasite. It might sound a little horrifying to consider it as such, but immunologically, biochemically and genetically speaking, an embryo is completely different from the endometrial cells. The maternal immune system should be rejecting them like any other parasite or bacteria. However, in successful implantation, i.e. when the embryo is of the adequate quality, the maternal immune system seems to be ignoring the embryo. This observation leads to a hypothesis that the embryos with a high potential of implantation would transmit a different message from a degenerated embryo.

Using the knowledge gained by the JAR spheroid-based embryo-maternal communication model, we have then tried to look for the differences in the messages transmitted from good quality and bad quality human embryo.

We used in vitro fertilized embryos. In vitro fertilized embryos are grown in bubbles of culture media. We collected the used or “conditioned” media from the embryos and isolated the extracellular vesicles released from the growing embryo. Then we supplemented the isolated EVs to endometrial cells. The results revealed that only the embryos that were of good quality were able to affect the pre-observed changes in endometrial cells.

These results are the foundation of a new understanding of the first communication between the embryo and the mother. A more comprehensive understanding would be useful in the future therapeutic and diagnostic applications.