Merkel cell carcinoma

Merkel cell carcinoma is an uncommon and very aggressive skin tumor. Carcinoma is the name for one type of cancer. Neuroendocrine, the other word used to describe it, combines nerve and hormone: it refers to cells that have features of nerve cells and of the cells that make hormones. It usually shows up on parts of the body that get sun exposure, especially the head and neck.

In most cases, a virus is behind this tumor: the Merkel cell polyomavirus. The virus slips its DNA, its own set of instructions, into the cell's genome, which is the complete instruction manual the cell runs on. Once it's there, the virus uses one of its own proteins, the large T antigen, to switch off the retinoblastoma protein. That protein is a tumor suppressor, one of the proteins that normally put the brakes on the cell cycle, the series of steps a cell goes through to get ready to divide. Without that brake, the cycle runs out of control, like a car parked on a hill when someone releases the parking brake: nothing holds it back anymore.

And here the name is a little misleading. Normal Merkel cells are mature skin cells that have already specialized. But this carcinoma is thought to arise not from them but from a skin stem cell, a cell that hasn't specialized yet and can turn into other kinds of cells. It's a question scientists are still debating.