Implantation is a milestone in human development as it is from this stage onwards that the embryo really begins to take shape and the overall body plans are decided.
- Magdalena Zernicka-Goetz
Once an egg has been fertilised by a sperm, it divides several times to generate a small, free-floating ball of stem cells. Around day three, these stem cells cluster together inside the embryo towards one side; this stage is known as the blastocyst. The blastocyst comprises three cell types: cells that will develop into the future body (which form the ‘epiblast’), cells that will develop into the placenta and allow the embryo to attach to the womb, and cells that form the primitive endoderm that will ensure that the fetus’s organs develop properly and will provide essential nutrients.
This pre-implantation period – so-called as the blastocyst has yet to implant itself into the uterus – has been extensively studied in human embryos using in vitro culture methods. However, on the seventh day of development, the human embryo must implant into the uterus of the mother to survive and to develop further, even though UK law permits embryos to be studied in the laboratory for up to 14 days.
The failure of an embryo to implant is a major cause of early pregnancy loss and yet the cellular and molecular changes that take place in the human embryo at this stage remain unknown. This is because it is impossible to carry out such studies on embryos developing in the womb, and until now there has been no system to culture human embryos in the laboratory beyond day seven.
In parallel papers in Nature and Nature Cell Biology, two international teams report the development of a technique that allows them to culture human embryos outside the body of the mother for an additional six days, up to day 13 of development. This work builds on previous work by Professor Magdalena Zernicka-Goetz’s team from the University of Cambridge on mouse and was funded by the Wellcome Trust.
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Image: Imaging a human embryo in the absence of maternal tissues - day 10 (left) and day 11 (right)
Credit: Zernick-Goetz lab, University of Cambridge
Reproduced courtesy of the University of Cambridge
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