Human embryos have been grown outside a womb for two weeks, doubling the previous record. This study, carried out so scientists could experiment, has ignited a new round of debate of the ethics of human engineering.

In vitro fertilization does not usually extend past the implantation stage — the time during which fertilized eggs would normally implant themselves into the womb. This new research marks the first time that human embryos have been grown in vitro past this stage.

"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 plan are [sic] decided. It is also the stage of pregnancy at which many developmental defects can become acquired. But until now, it has been impossible to study this in human embryos," Magdalena Zernicka-Goetz of the University of Cambridge said.

Following fertilization, human eggs start to divide into a small ball of stem cells, each capable of developing into any type of cell. Within three days, these cells begin to move to specific locations, a period where the developing bundle is known as a blastocyst. At this point, the cells are composed of the epiblast, which will form the body, an endoderm, essential for the development of organs, and cells that will form the placenta.

One week after fertilization, the cluster must implant into a womb in order to survive. This problem prevented researchers from studying embryos in the laboratory past this early stage of development. However, laws in the United Kingdom (UK) allow researchers to study these bodies up to two weeks after fertilization.

Soon after implantation, embryos develop a cavity, which develops into the fundamental structure of the developing body. Previously, medical researchers believed this cavity was the result of apoptosis, or programmed cell death. This new investigation reveals this process is not needed for the developing body of cells.

Study of how developing embryos change between one and two weeks could allow investigators to learn more about miscarriages. A failure to implant in wombs is a leading cause of early accidental terminations.

Development of techniques to extend the life of embryos past the implantation stage and examination of how they develop was profiled in Nature Cell Biology.