Cells of origin and tumor-initiating cells for nonmelanoma skin cancers.

The epidermis of the skin is a multilayered stratified epithelium whose primary function is to provide a barrier against our external environment. As a result, cells in the epidermis are subject to constant assault from environmental pathogens, many of which can cause deleterious mutations. However, most of these mutations do not lead to skin cancer. One explanation is that most genetic hits are sustained by mature or transit cells with limited proliferative capacity and only stem cells that acquire genetic alterations have the potential to propagate a frank tumor. In this mini-review we will discuss recent studies that provide some of the first genetic evidence to support a stem cell origin for a number of skin cancer types.