Researchers have shed light on an important stage of early embryo development that has never been fully mapped out in humans before.
Because of the nature of available samples, studies so far have focused on the first week after conception.
They have also looked at later stages beyond a month into pregnancy, during which organs form and mature.
But there is currently very little understanding of what happens in the days in between.
This includes the gastrulation stage that occurs shortly after the embryo implants in the womb.
During this process an embryo transforms from a one-dimensional layer of cells and reorganises into a multi-layered and multidimensional structure called the gastrula.
It is the first time an embryo at this stage of development has been characterised in such detail using modern technology
Dr Richard Tyser
Experts say gastrulation is one of the most critical steps of development, and takes place roughly between days 14 and 21 after fertilisation.
The three main cell layers that will later give rise to the human body’s tissues, organs and systems are formed during this stage.
Principal investigator Professor Shankar Srinivas said: “Our body is made up of hundreds of types of cells.
“It is at this stage that the foundation is laid for generating the huge variety of cells in our body – it’s like an explosion of diversity of cell types.”
Scientists say the study is a milestone for developmental biology as ethically obtained human samples at these early stages are exceptionally rare.
The sample was obtained through the Human Developmental Biology Resource, from an anonymous donor who provided informed consent for the research use of embryonic material arising from the termination of her pregnancy.
The sample is estimated to be from around 16 to 19 days after fertilisation.
Lead researcher Dr Richard Tyser said: “This is such an early stage of development that many people would not have known they were pregnant.
“It is the first time an embryo at this stage of development has been characterised in such detail using modern technology.”
Legally, scientists are only able to culture human embryos up to the equivalent of 14 days of development – just before the start of gastrulation.
Therefore the knowledge of events beyond 14 days after fertilisation is largely based on studies in animal models such as mice and chickens.
Dr Tyser said: “Our new sample is the bridge that links the very early stage of development with the later stages when organs begin to form.
“This link in the human had previously been a black box, so we had to rely on other model organisms such as the mouse.
“Reassuringly, we have now been able to show that the mouse does model how a human develops at the molecular level.
“Such models were already providing valuable insights, but now this research can be further enriched by the fact we’re able to cast light into that black box and more closely see how it works in humans.”
Speaking at a press briefing, Prof Srinivas said researchers did not see any neurons in the sample.
He said: “We didn’t see any differentiated neurons to the sample, which tells us that the human embryos at this earlier stage are not equipped to sense their environment in many ways, and certainly can’t by any stretch of the imagination be said to be have any mechanism for consciousness for example – the cells just aren’t there.”
Prof Srinivas added that while the researchers do not take any position on the 14-day rule in the study published in the journal Nature it does “provide data which will inform the debate and the discussion about whether we want to change the 14-day rule and whether it’s right”.
Researchers were able to identify 11 distinct cell types, and while most of these cells were still immature, they discovered the presence of both blood cells and the primordial germ cells that give rise to gametes (ovum and sperm cells).
The findings will contribute to the improvement of experimental stem cell models.