Blood Watch

Short answer: Intravital microscopy means looking at living tissue inside a living animal, and blood flow was one of the first things it revealed. In 1661, Marcello Malpighi used a microscope on a frog's lung and described capillaries, the tiny vessels linking arteries and veins.

In 1839, Rudolph Wagner watched white blood cells roll slowly along vessel walls in a frog's webbed foot. Thin, transparent tissues such as a frog's web, a tadpole's tail or a fish's fin let ordinary bright-field microscopes show red blood cells streaming through capillaries in real time. Plasma, the liquid part of blood, is transparent, so it can't be seen directly. Researchers make it visible by injecting fluorescent tracers, such as FITC-dextran, that stay in the bloodstream and light up the vessels. Modern intravital imaging relies on fluorescence. Labeled antibodies, dyes or genetically encoded fluorescent proteins mark particular cells, and confocal and two-photon microscopes take sharp optical sections. Two-photon microscopy reaches deeper into tissue with less light damage, which is why it's widely used to film immune cells in living mice. In mammals, the small cells that help blood clot are platelets. Fish, amphibians and birds have nucleated thrombocytes instead, and their red blood cells also keep a nucleus. The footage shows transparent tissue with dark pigment cells and branching vessels where blood cells can be seen moving. The source doesn't identify the animal.

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