Cell structure. Confocal light micrograph of cultured endothelial cells. A fluorescent dye has been used to show the cell structure. Nuclei (blue) contain the cell's genetic information, DNA (deoxyribonucleic acid) packaged in chromosomes. Actin filaments (green) and tubulin (red), which forms microtubules, part of the cell cytoskeleton are also seen. The cytoskeleton is responsible for intracellular transport, structure and motility of the cell, as well as segregating the chromosomes during nuclear division. Actin is the most abundant cellular protein. Endothelial cells are flat and line all of the body's blood vessels.