Culture cell. Computer-enhanced confocal light micrograph of a 3t3 (mouse fibroblast) cell in culture. The nucleus is pink, and the cell's cytoskeleton is green. The cytoskeleton is a network of structural proteins that supports the cell's organelles. 3t3 cells are an immortal cell line widely used in biological research. Magnification: x1000 when printed 10cm wide.