This shot of the dorsal neck of a developing mouse embryo visually shows the process of skeletal growth found in all bony vertebrates. The widespread blue stain (Alcian blue) reveals the extensive cartilaginous blueprint for the growing skeleton, which extends by the division of chondrocytes (cartilage-producing cells) guided by signalling molecules across the embryo. The conversion of the cartilage to ossified bone is shown here by the red stain (Alizarin red), beginning in the centre of the cartilage model and extending outwards to the distal ends, the tips of which are still visible in blue.

Image courtesy of the Kusumi Lab at Arizona State University.