Koilocyte

A koilocyte is a cell that has changed shape because of an infection with human papillomavirus, or HPV. The name comes from the Greek word koilos, meaning "hollow," and it describes exactly what you see: a cell from the squamous epithelium, the layer of flat cells that lines some surfaces of the body, with a hollow space around its nucleus. The nucleus is the cell's control center, where its DNA is kept.

Here is what happens inside. The virus gets in through the basal cells, the youngest cells, which sit at the bottom of the epithelium. As those cells mature, the virus multiplies inside them. One of its proteins, called E4, scrambles the cytoskeleton, the internal framework that gives the cell its shape. Picture a sunny-side-up egg seen from above: the yolk is the nucleus, and the white is the cytoplasm, the material that fills the cell. In a koilocyte, it's as if someone scooped out the white right around the yolk, leaving an empty ring. The yolk also gets bigger, its chromatin (the material that DNA is packed into) looks coarse, and its edge turns irregular.

You can't see these cells with the naked eye, only under a microscope, and they often turn up on a Pap smear, the test that looks at cells collected from the cervix. Because they are specific to HPV, finding them is like finding the virus's fingerprint in the tissue. And HPV infection is often temporary.