What's the difference in skin cells? Melanocytes and basal, squamous and Merkel cells

skin cancer
The skin is the body's largest organ, and like other organs, it can develop cancers. Skin cancer is the most common cancer in the United States. But treatments and prognoses vary greatly, depending on the affected cells. Learn more

Consider the remarkable functions the skin performs every day. It's only 1.5 millimeters at its thickest point, but it protects the muscles and organs from outside threats. It can take a pounding, enduring bumps and bruises, the sun’s burning rays and the grime left by dirt and dust. It moves and stretches when you do, but even when the body is at rest, the skin is a bustle of cellular activity. Basal cells are changing shape as they move to the surface to replace dying squamous cells. Merkel cells sense the touch of the sheets or your clothes. And melanocytes are producing melanin, the skin-darkening pigment designed to protect the skin from the sun.

The skin is the body's largest organ, and like other organs, it can develop cancers. Skin cancer is the most common cancer in the United States, affecting more than 3 million people a year. But treatments and prognoses vary greatly, depending on the affected cells. "Being diagnosed with a skin cancer is really a meaningless term," says Diego Muilenburg, MD, a Surgical Oncologist at our Phoenix hospital. "It really matters if it's a squamous cell or basal cell carcinoma or a Merkel cell cancer or melanoma. The treatment paradigm is completely different for each."

Skin cells and skin cancers

Most of the cells found on the epidermis, the outer layer of the skin, are basal cells, also called keratinocytes. These round cells form at the bottom of the epidermis and flatten as they make their way to the surface, transforming into squamous cells. Basal cell carcinomas account for about 80 percent of all skin cancers, according to the American Cancer Society. Most of the cells on the skin's surface are flat, scale-like squamous cells. These cells are also found in the throat, the linings of the respiratory and digestive systems and in hollow organs, such as the kidneys. Squamous cell carcinomas account for about 20 percent of all skin cancers. Basal cell and squamous cell skin cancer cells grow slowly, they usually don't metastasize, and they are rarely fatal. But both types of cancer, especially squamous cell carcinomas, may cause serious complications if left untreated. They often appear as a growth, bump or sore that doesn't go away.

"Basal cell and squamous cell skin cancers are pretty common," Dr. Muilenburg says. "They tend to show up on sun-exposed areas—the face, legs and arms. And on the skin, they are often treated the same way—a dermatologist will freeze or surgically remove the affected skin, and you don't have to worry about them too much unless they are completely neglected. With melanomas and Merkel-cell cancers, it's dramatically different."

Merkel cells, named for the 19th-century German physician who originally described them, are found below the epidermis, nestled next to nerve cells. They are sometimes called touch cells because they help the skin feel light touch, textures and other fine details. Merkel cell carcinomas may appear as bumps or nodules on sun-exposed skin. This is a rare cancer, but it is often aggressive and may metastasize. In March, the U.S. Food and Drug Administration approved the immunotherapy drug avelumab (Bavencio®) to treat metastatic Merkel-cell carcinoma. Melanoma is an aggressive form of skin cancer that develops in the melanocytes. Only 2 percent of all skin cancers are melanomas, but they account for most skin-cancer deaths. "Even a tiny melanoma can spread to your lymph nodes and grow like wildfire throughout the body," Dr. Muilenburg says. "It's such an insidious cancer with a high propensity for recurrence. Surgeons typically remove melanoma by excising it from a wide area that may require skin grafts or reconstructive surgery to repair. On top of that, the surgeon needs to determine if the cancer has spread into the lymph nodes. It can spread practically anywhere."

Know your ABCs

It may be difficult to determine whether a mole, bump or sore has developed into cancer, or which cancer. Dr. Muilenburg recommends using the ABCDE test to help evaluate whether an abnormal growth may be melanoma:

  • A is for asymmetry: Does the mole have an irregular shape?
  • B is for border: Is it rough or notched around the edges?
  • C is for color: It is changing shade or color?
  • D is for diameter: Is it larger than the size of a pencil eraser?
  • E is for evolution or elevation: Is the spot growing in height or width?

"If it's changing, if the border is irregular, if it is getting crusty or bleeding, you should have it looked at," he says. "And if you have any doubts, go see a dermatologist."