13.5 days after fertilization of the egg, the mouse embryo is about 1cm head to tail, and about ⅔ done with gestation. At this point the motor nerves (green), which will connect the brain (the spinal cord) to every muscle, are send their axons out to find the developing muscles. The sensory nerves (red) are projecting from the dorsal root ganglia (flanking the length of the spinal cord on both sides) are sending axons out to the body as well. All of this is visualized by dissolving fats, which scatter light, and matching the refractive index of the sample with organic solvents (an old technique) which makes the embryo appear virtually transparent. Then laser light sheet microscopy (a new technique) is used to image the entire embryo at very high resolution.
Soundtrack - Ashes to Ashes, David Bowie