Scientists have grown “complete” models of human embryos in the lab with all the known features found in normal embryos around two weeks old.

Known as stem cell-based embryo-like structures, or SEMs, they were developed without using sperm, eggs or a womb.

The researchers at the Weizmann Institute of Science in Israel said their models contained hormones that caused a commercial pregnancy test to turn positive.

They stated that unlike previous embryo models, theirs have shown the ability to progress to the next developmental stage, offering “an unprecedented opportunity to shed new light on the embryo’s mysterious beginnings”.

It closely mimics the development of a real human embryo, particularly the emergence of its exquisitely fine architecture

The team are hoping their work could shed light on the “drama” that occurs in the first weeks of human development and help gain insight into what causes birth defects or why miscarriages occur.

Professor Jacob Hanna who led the research, said: “The drama is in the first month, the remaining eight months of pregnancy are mainly lots of growth.

“But that first month is still largely a black box.

“Our stem cell–derived human embryo model offers an ethical and accessible way of peering into this box.

“It closely mimics the development of a real human embryo, particularly the emergence of its exquisitely fine architecture.”

For the study, published in the journal Nature, the researchers used naive stem cells – which have the potential to become any type of tissue in the body.

Chemicals were then used to coax these cells to grow.

The cells then arranged themselves into structures that mimicked the features of a real human embryo.

The models were allowed to grow until they reached a stage equivalent to a two-week old embryo after fertilisation.

Prof Hanna said: “An embryo is self-driven by definition; we don’t need to tell it what to do – we must only unleash its internally encoded potential.

“It’s critical to mix in the right kinds of cells at the beginning, which can only be derived from naive stem cells that have no developmental restrictions.

“Once you do that, the embryo-like model itself says ‘Go!'”

In many countries, 14 days is the legal cut-off for normal embryo research.

However, these “embryo models” are not legally seen as embryos and are not governed by the same laws.

The researchers said this approach could help reveal the causes of many birth defects and types of infertility.

It could also lead to new ways of growing transplant organs as well as offer a way around experiments that cannot be performed on live embryos, the team added.

Prof Hanna said: “Many failures of pregnancy occur in the first few weeks, often before the woman even knows she’s pregnant.

“That’s also when many birth defects originate, even though they tend to be discovered much later.

“Our models can be used to reveal the biochemical and mechanical signals that ensure proper development at this early stage, and the ways in which that development can go wrong.”

This research and other recent reports on models of the early human embryo show that models of human embryos are getting more sophisticated

Commenting on the study, Profesor Alfonso Martinez Arias, from the department of experimental and health sciences at Pompeu Fabra University in Spain said he expected the work to raise ethical issues but he added it was “a most important piece of research”.

He said: “The work from the Hanna lab just published has, for the first time, achieved a faithful construction of the complete structure from stem cells, in vitro, thus opening the door for studies of the events that lead to the formation of the human body plan.”

Darius Widera, professor of stem cell biology and regenerative medicine at the University of Reading, said: “Despite being a significant stepping stone, this study is focused on early embryo development, and the protocols have a relatively low efficacy.

“Importantly, although the embryo-like structures resemble early human embryos, they are not identical.

“This research and other recent reports on models of the early human embryo show that models of human embryos are getting more sophisticated and closer to events that occur during normal development, highlighting that a robust regulatory framework is more needed than ever before.”