Fibroblast cells. Confocal light micrograph of mammalian fibroblast cells. The cells have been stained to reveal their structures: nuclei purple, and cytoskeletons yellow. Fibroblasts are found in connective tissue. They are responsible for producing collagen, the main structural protein in the body. In confocal microscopy a laser excites fluorescent dyes placed in the specimen. Magnification: x400 when printed 10cm wide.