The nucleus is the part of the cell that contains most of your genetic material (DNA). The nucleus is located within the cytoplasm of the cell. The combination of DNA and other helper proteins found inside the nucleus is called chromatin. The nucleus is surrounded by a thin barrier called the nuclear membrane.
Two dyes called hematoxylin and eosin (H&E) are added to a tissue sample before it is examined under the microscope. The hematoxylin goes into the nucleus which gives it a purple or blue colour. In most cells, the nucleus has a circular or oval shape.
The look of the nucleus when examined under the microscope can tell a pathologist a lot about the behavior of the cell. For example, very active cells may group their genetic material into round balls called nucleoli which can be seen inside the nucleus. Cancer cells also tend to use their genetic material more than normal cells which makes their nucleus darker and larger. Pathologists call dark nuclei hyperchromatic. In non-cancerous cells, the nuclear membrane is normally smooth and round. In cancer cells, however, the nucleus loses its round shape and folds can be seen in the nuclear membrane. Pathologists describe these changes as nuclear membrane irregularities.
The size and shape of the nucleus can also be changed by viruses and radiation, both of which cause the nucleus to become much larger than normal. Viral cytopathic effect is a term pathologists use to describe the abnormal nucleus in a cell that has been infected by a virus.