An epidermoid cyst is a non-cancerous growth that develops in the dermis just below the surface of the skin. It is a round, hollow structure lined by the same squamous cells found in the epidermis. Epidermoid cysts appear as small bumps, usually 1-4cm in diameter. While they arise most frequently on the face, neck, and torso, they can arise at any skin site. Epidermoid cysts are sometimes referred to as ‘epidermal cysts’, ‘infundibular cysts’, or ‘epidermal inclusion cysts.’
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.
No one really knows why epidermoid cysts occur. On some areas of the body, like the palms and soles, epidermoid cysts may be caused by trauma that pushes some of the epidermis into the dermis. At other body sites, epidermoid cysts are thought to arise from inflammation of the hair follicle.
The diagnosis is usually made after the entire cyst is surgically removed and sent to a pathologist for examination under the microscope. Pathologists look for a round structure lined by keratinizing squamous epithelium. The squamous epithelium shows a kind of maturation similar to that seen on the skin surface. The inside of the cyst may be filled with keratin debris which looks similar to the keratin found on the surface of the skin.
Epidermoid cysts may break open or rupture. This can lead to pain and inflammation as keratin from inside the cyst spills into the surrounding tissue. When examined under the microscope, your pathologist may see specialized immune cells called multi-nucleated giant cells and other chronic inflammatory cells in the dermis around the ruptured cyst.