A small tropical fish that lets us watch blood and lymphatic vessels assemble, live, inside a developing animal.
The zebrafish (Danio rerio) is one of the most powerful models in vascular biology. Its embryos and larvae develop externally and are almost completely transparent, so an entire vascular network can be imaged as it forms in a living animal — at a resolution and over a timescale that simply aren't possible in mammals.
Just as importantly, the genetic toolkit is powerful. Transgenic lines light up endothelial cells and lymphatics in fluorescent colour, CRISPR/Cas9 makes gene editing fast and precise, and the core pathways that build vessels are conserved with humans. Large clutches and rapid development make zebrafish ideal for genetic screens and for asking what happens when vascular development goes wrong. That combination is exactly why we use them to study the vasculature in health and disease.