Keratinocytes form at the bottom layer of the epidermis. Over a period of about 30 days, they push their way to the outer epidermal layer. As young keratinocytes migrate to the skin's surface, they displace their older, dying counterparts, which flake off or are brushed or washed away. In this way, humans literally shed their skin.
Most people lose a layer of keratinocytes every month or so.
They contain melanin, the protein that determines skin color. In general, the more melanin, the darker the skin. But whether a person's skin is light or dark depends on various factors, in particular inherited genes that direct the quantity and quality of melanin production. Scientists believe excessive sun exposure, along with genetics, may play a role in the type of skin cancer known as melanoma, where melnocytes become cancerous and produce a tumor. If the melanoma tumor spreads to vital organs, such as the heart, lung, brain, or liver, death can result.