Cell nuclei. Immunofluorescent light micrograph of cultured fibroblast cells showing nuclei with nucleolar necklaces". The round structures are the nuclei, the control centres of the individual cells. Within the nuclei are the nucleoli. These have been altered structurally in preparation to appear as ring-like structures, or "nucleolar necklaces". These are the sites of ribosome RNA synthesis and ribosome assembly. Dyed green in the cytoplasm is the cytoskeletal protein actin. This helps to maintain the shape of the cell. Immunofluorescence is a technique using antibodies to attach fluorescent dyes to cells. Magnification: x624 at 6x4.5cm size."