A signet ring cell is a specialized type of cell that is round and filled with a light blue coloured material called mucin. Pathologists can recognize these type of 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 ring 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 ring cells are important because some cancers are made up mostly or entirely out of signet ring cells. For example, these cells are commonly seen in cancers of the stomach, ovary, and breast.
Some other types of cells may look very similar to signet ring cells and special tests, such as special stains or immunohistochemistry may be required to confirm the presence of true signet ring cells. A common special stain to look for signet cells is called mucicarmine and it makes the mucin inside the cell easier to see.