The nucleolus is a round body found in the nucleus of most cells. Although it is not technically an organelle, since it is not bound by a membrane, it can still be seen using an electron microscope and plays an important role in... A. Regulating substances that enter and exit the nucleus. B. Synthesizing ribosomes and ribosomal RNA. C. Synthesizing mitochondria and mitochondrial DNA. D. Producing the fluid in the nucleus of cells.