Understanding human embryo implantation: A research breakthrough

2025 concluded with the simultaneous publication of three studies – by Matteo Molè and colleagues, by Qian Li and colleagues and by Jinzhu Song and colleagues – exploring how early human embryos, and also stem cell-based embryo models (SCBEMs), interact with human endometrial tissues in culture (see BioNews 1321).

These studies represent important advances in our understanding of human embryo implantation, which is a critical and complex aspect of our early biology that has been largely intractable for researchers until now. There is substantial overlap between the three studies, which adds considerable robustness to their findings.

In each study, endometrial tissues were derived from biopsies of the surface layers of a woman's womb, and were then grown as 3D structures. In each study, human embryos appeared to implant in these endometrial structures in a biologically meaningful way, mimicking what is suspected to happen when a pregnancy is established following natural (or assisted) conception.

At present, we know relatively little about embryo implantation in humans. Much of what we do know has been learned from fixed specimens, which can only ever provide a static picture. Nor can we rely on what is known to happen in research animals, because aspects of embryo development – and particularly of the placenta – show substantial differences in gene activity (and even in cell types) when animal species are compared with humans or with one another.

The new studies made use of embryos donated to research by IVF patients (see BioNews 1219 and 1243). Experiments involving such embryos were terminated before embryo development exceeded 14 days, so as to stay within laws and guidelines – in the UK and elsewhere – that specify a 14-day limit.

Embryos that implanted in the endometrial structures seemed to develop very well up to this 14-day limit. Indeed, these embryos seemed much more normal – in terms of both morphology and gene activity – than embryos that have been cultured, in these laboratories and in several others, for a similar period of time within a non-cellular 3D matrix.

This success could imply the presence of signals between the embryos and the endometrial cell types, with each promoting the development of the other. Alternatively (or additionally), this could imply that better structural support from the endometrial tissue helps the embryo to remodel the environment to its own needs, while perhaps also enabling a better supply of nutrients and better removal of waste products. Some of the findings by Molè and colleagues are consistent with the former explanation, while not excluding the latter explanation.

Another notable finding of these studies is that SCBEMs seem to be able to implant in endometrial tissue in a similar way to natural human embryos, which bolsters existing evidence that SCBEMs are successful at modelling relevant aspects of embryos. Li and colleagues exploit this fact in their study – they found that SCBEMs are not as successful at implanting in endometrial tissue, if that tissue originates from women who have experienced repeated implantation failure. The researchers were therefore able to use their system as an assay, to screen for drugs that could potentially help to overcome this problem.

It is interesting to consider how embryos cultured with endometrial tissues in this way would fare, if they were allowed to develop beyond 14 days (see BioNews 1097). This a step which could only be pursued by UK researchers if there were a change to UK law, as has been recommended by the UK's regulator of fertility treatment and embryo research – the Human Fertilisation and Embryology Authority (see BioNews 1268 and 1269) – and also by many others (see BioNews 1083, 1086, 1152, 1214, 1221 and 1296). There is evidence to suggest that the UK public would be supportive of such a change (see BioNews 885 and 1213).

If the current 14-day limit were extended, this would enable scientific research into critical stages of embryo development that are not well understood. An extension of the 14-day limit could also enable further validation of SCBEMs, such that SCBEMs could replace natural human embryos in certain experiments without loss of confidence in the meaningfulness of the results.

That said, it is important to note that SCBEMs might not continue to develop in a way that emulates the development of natural human embryos (see BioNews 1292). Indeed, it seems unlikely that SCBEMs could do this indefinitely. None of the different types of SCBEM that have been created to date undergoes the early events that occur in embryos shortly after fertilisation, and these events may be essential for programming gene activity that will occur during later stages of development. Just as extension of the 14-day limit could help us to validate SCBEMs, it could also help us to understand whether and when such validation fails.

Another important point – which has perhaps been missed in some of the media coverage of the new studies – is that while coculture systems with endometrial cells might permit investigation of the first few weeks of post-implantation development, this does not amount to ectogenesis (see BioNews 1302). The endometrial tissues used in these studies lack other critical cell types found in the womb, with the lack of a vasculature system – which is essential for efficient oxygen and nutrient delivery, once the embryo begins to grow more rapidly – being especially significant.

Complete ectogenesis – the development to viability of an organism entirely outside a host organism – would be a far more formidable (and ethically contentious) prospect. What these new studies have demonstrated is not so much a step towards ectogenesis, as it is a breakthrough in establishing promising new approaches to the treatment or avoidance of fertility problems (notably implantation failure and early miscarriage) and congenital disorders.