Scientists have created a near-complete mouse embryo out of stem cells.

The embryo, which was developed by a team from the Thisse Lab at the University of Virginia School of Medicine, contains blood vessels, muscles and even a beating heart according to an article on New Atlas.

This is a process that has taken years to build and perfect, with coaxing and timing the cells to develop properly being one of the biggest hurdles to overcome. In the past, the team have created zebrafish embryos, before eventually developing the stem cell-based mouse embryo, which is reportedly a first of its kind.

“We found a way to instruct aggregates of stem cells to initiate embryonic development,” said Christine Thisse, a scientist behind the study.

“In response to this controlled instruction, the aggregates develop into embryo-like entities in a process that recapitulate the embryonic steps one-by-one.”

The mouse embryo is still missing some pieces such as the anterior region of the brain, though another scientist behind the study Bernard Thisse states that the team will be able to rectify and master this over time.

“The embryoids we are currently producing lack the anterior brain domains. However, with the techniques we have developed, we should be able, at some point, to manipulate molecular signals that control embryo formation, and this should lead [to] generating embryo-like entities containing all tissues and organs including the anterior brain,” said Thisse.

The ultimate aim of this research is to create functional human organs that can be used for transplants.