Up until now, the longest that human embryos have been grown in the laboratory was nine days, though seven days has been far more common.
Now, two teams of researchers in the US and the UK have created an experimental system that allows embryos to be cultured outside the uterus in the complete absence of maternal input for up to 13 days.
We can now, for the very first time, study human development at this very critical stage of our lives, at the time of implantation. This work also raises the possibility that scientists could soon culture embryos to an even more advanced stage.
The advance should help scientists investigate many longstanding questions, including: Why do so many pregnancies end in miscarriages? How could infertility treatments be improved? What causes birth defects? How do embryonic stem cells really work?
This research has been possible thanks to couples that underwent IVF treatment and decided to donate their surplus embryos to advance our understanding of the early phases of post-implantation human development.
A normally fertilized egg should divide into 4 cells within 48 hours, the day after, into 8 cells, and by the fifth to seventh day after fertilization, the embryos should reach the blastocyst stage (80 or more cells).
The teams watched as the cells in the embryos began to differentiate and reveal features that are unique to human development. For instance, a group of cells showed up in the embryo around day 10 and disappeared around day 12. The scientists do not yet know the function of this cell cluster, but it seems to be a transient organ, akin to the tails that human embryos grow much later in development and then lose before birth.
In accordance with internationally recognized bioethical guidelines, the groups’ experiments were concluded on day 14 post fertilization, well before even the earliest signs of nervous system development are observed.
The ability to grow an embryo in vitro for 13 days raises ethical and policy considerations. At least 12 countries, including the United Kingdom, bar scientists from working with embryos older than 14 days. The US government drew up guidelines suggesting the same limit in 1979.
The scientists expressed doubt that their embryos would survive much beyond the 14-day mark, because work in mice suggests that more developed embryos need an unknown mix of hormones and nutrients from the mother to survive.
On the other hand, if materials science advances as fast as biology, the ability to develop advanced biomaterials to build an artificial womb may not be that far off.