Loading video... VIDEO: This video shows a beating mouse heart crafted from stem cells at the University of Virginia School of Medicine. The work represents a major step forward in scientists' efforts to... view more Credit: Courtesy Thisse lab at UVA. The tiny mouse embryo has a heart that beats. Its muscles, blood vessels, gut and nervous system are beginning to develop. But this embryo is unusual: It was made in a lab, out of mouse embryonic stem cells, and represents the most sophisticated in vitro (in a dish) model of a mammal ever so created. This new model, developed at the University of Virginia School of Medicine by Christine and Bernard Thisse, is a major step forward in scientists' efforts to mimic the natural development of a mammal by using stem cells. Its existence is a wonder that will help scientists understand mammalian development, battle diseases, create new drugs and, eventually, grow tissues and organs for people in need of transplants.