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Human endothelial cells. Light micrograph of cultured human endothelial cells, a cell type found lining the blood vessels. Fluorescent dyes have been used to show the cell structures. Actin filaments are red, the cell nuclei are blue, and microcorpuscules containing Von Willebrand factor are yellow. Actin is the most abundant cellular protein, forming part of the cells' cytoskeleton, the system that is responsible for intracellular transport, and the structure and motility of the cells. The cell nuclei contain the cells' genetic information, DNA (deoxyribonucleic acid), packaged in chromosomes. Von Willebrand factor is a blood clotting protein. Magnification: x350 when printed at 10 centimetres wide.