The nucleus is the part of the cell that contains most of your genetic material (DNA). The combination of DNA and other helper proteins found inside the nucleus is called chromatin. The genetic material is kept inside the nucleus by an outer wall 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, cancer cells 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 membrane of the nucleus is smooth and round. In cancer cells, however, the nucleus loses its round shape.
Very active cells may also group their genetic material into round balls called nucleoli which can be seen inside the nucleus.
The size and shape of the nucleus can also be changed by viruses and radiation, both 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.