A new microscope has given researchers their most detailed view yet of the earliest days of mouse development. For the first time, scientists can track each cell as it divides, from the fertilized egg all the way to the round ball of cells called a blastocyst, which implants in the mother’s uterus (see video, above). Mammalian embryos, including mice and humans, are very sensitive to the bright light used in standard high-resolution microscopes. A technique called light-sheet microscopy illuminates just a thin slice of the embryo at a time, minimizing the living specimen’s exposure. Researchers have now designed a light-sheet microscope that can image multiple embryos at once, keeping them at a stable temperature in their growth media. That allows the scientists to average out the differences between individual embryos and better recognize meaningful patterns in development. They report this week in Nature Methods that in mouse embryos, the first differentiation of cell types happens when the embryo grows from eight to 16 cells. When the cells in the eight-celled embryo divide, cells that end up on the inside (tagged red in this video) continue to develop into the body of the mouse, while those on the outside (tagged blue) go on to form the placenta. The technique could also be used on human embryos, the researchers say, and might help settle a long-running dispute on exactly when cells in the embryo start to differentiate into specific cell types.

(Video credit: European Molecular Biology Laboratory)