Seborrheic keratosis (SK) is a non-cancerous growth that starts from the squamous cells in the epidermis on the surface of the skin. Seborrheic keratosis is a very common condition and is seen more often as people age.
Without a microscope, seborrheic keratosis may look like a raised or bumpy area of skin with a light tan or black colour. It is often described as looking as if it was “stuck on” to skin. The size of the involved skin can range from a few millimeters to a couple of centimeters and many patients have more than one. It may look similar to a wart or liver spot.
Skin is made up of three layers: epidermis, dermis, and subcutaneous fat. The surface and the part you can see when you look at your skin is called the epidermis. The cells that make up the epidermis include squamous cells, basal cells, melanocytes, Merkel cells, and cells of the immune system. The squamous cells in the epidermis produce a material called keratin which makes the skin waterproof and strong and protects us from toxins and injuries.
The dermis is directly below the epidermis. The dermis is separated from the epidermis by a thin layer of tissue called the basement membrane. The dermis contains blood vessels and nerves. Below the dermis is a layer of fat called subcutaneous adipose tissue.
The diagnosis is usually made after a small area of skin is removed in a procedure called an excisional biopsy or an excision. The tissue is then sent to a pathologist who examines it under a microscope.
When examined under the microscope, seborrheic keratosis is made up of small, round, and flattened squamous cells. Small spaces called cysts filled with keratin are also seen in the epidermis. Pathologists call these spaces “keratin pearls” because they are filled with rings of keratin. Your pathologist will examine the tissue sample carefully to make sure there is no sign of cancer or pre-cancerous disease.
Pathologists divide seborrheic keratosis into types based on how the cells look under the microscope. Each type is called a variant.
The most common variants of seborrheic keratosis are: