Our hearts began beating as early as four weeks after conception, even before they had fully developed. Heart valves formed shortly afterwards, ensuring blood always flows in the right direction – a similar valve is developing in this beating zebrafish heart. By recording layers of endocardial cells lining the zebrafish heart (artificially coloured grey here), researchers were able to assemble this 3D video of two consecutive heartbeats. Looking a bit like a school of swarming fish, the endocardial cells reveal the shape of the heart chambers pumping blood between the atrium (outlined on the right) and ventricle (on the left). Later in the video, the second heartbeat highlights cells in the developing valve (known as valve leaflets) in bright colours. The leaflets come together to close the opening between the atrium and ventricle, the atrioventricular canal – a developmental milestone that allows life to progress past this delicate early stage.
Written by John Ankers
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