A signet cell is a special type of cell that is round and filled with a light blue coloured material called mucin. Pathologists can recognize signet cells under the microscope because the mucin pushes the nucleus of the cell to the side which makes the cell look like a ‘signet ring’.
Signet cells are often found in groups, however, unlike other types of cells they do not stick together. Pathologists call cells that do not stick together discohesive.
Signet cells are important because some cancers are made up mostly or entirely out of signet cells. For example, signet cells are commonly seen in cancers of the stomach, ovary, and breast.
Some other types of cells may look very similar to signet cells and special tests, such as special stains or immunohistochemistry may be required to confirm the presence of true signet cells. A common special stain to look for signet cells is called mucicarmine and it makes the mucin inside the cell easier to see.
Signet cells are also called signet ring cells.