This video is awesome for so many reasons, I just have to post it here so you can watch it. Although it’s not a brand new video (it came from a 2002 Harvard study), it was a game-changing video for reasons I’ll describe in a minute. Plus it actually shows a clot forming in real time, in vivo (in a mouse). So it bears watching.

These two researchers from Harvard (Bruce and Barbara Furie) figured out how to label platelets (red), tissue factor (green), and fibrin (blue) with fluorescent antibodies so you can see what’s happening as a clot forms. The colors tend to overlap, but you get the idea. They actually used a high-speed digital video microscope (fancy!) to look through the paper-thin skin of a mouse’s scrotum and film the process. Amazing.

Check it out: the first thing that happens is that a platelet plug forms (see the red accumulating? The first little clump washes away, but right away some more clumps start forming). At the same time, tissue factor (green) shows up. Tissue factor is the thing that kicks off the coagulation cascade in vivo. At about 20 seconds, you start seeing fibrin (blue). 20 seconds! Also note how the clot gets pretty big – but remodeling cuts it back to a decent size by the end. Cool.

The video is important for another reason (besides its coolness factor). Before this research, the origin of tissue factor was somewhat mysterious. It was said to come from non-vascular cells, and it was thought to leak into the vascular system in cases of vascular injury. Hmmm. Sounds sketchy, but nobody had any better ideas.

In this video, you can see tissue factor showing up right away and staying mostly at the interface between the clot and the vessel wall. This supports the idea that tissue factor may be derived right from endothelial cells and also from tiny microparticles (which the Furies also discovered) which float through the blood, sticking to platelets and endothelial cells only when needed in the early stages of clot formation.