Pilar cyst

What is a pilar cyst?

A pilar cyst is a non-cancerous growth that starts from a hair follicle in the skin. They are very common and most start on the scalp where they look like a smooth, round bump on the skin. Pilar cysts range in size from a few millimeters to several centimeters. Another name for a pilar cyst is a trichilemmal cyst.

Skin

The outside of our body is covered in skin. The surface of the skin is called the epidermis. The epidermis is made up of keratinocytes which produce a material called keratin. The epidermis is waterproof and protects us from toxins and injuries. Below the epidermis is a layer called the dermis. The dermis contains important structures such as hair follicles, sweat glands, nerves, and blood vessels.

The hair follicles in our skin make hair. The follicle begins at the surface of the epidermis and is divided into different segments as it extends deep into the dermis. The parts of the follicle include the infundibulum, isthmus, and hair bulb.

How do pathologists make this diagnosis?

The diagnosis of pilar cyst may be made after a small sample of tissue is removed in a procedure called a biopsy. The diagnosis can also be made after the entire cyst is removed and sent to a pathologist for examination. No further treatment is usually required after the entire cyst has been removed.

When viewed through the microscope, most pilar cysts are filled with compact, ‘wet’ looking keratin. The keratin is surrounded by a layer of flattened keratinocytes without the usual granular layer.

Pilar cysts can rupture which means they break open and spill their keratin into the surrounding normal tissue. When this happens, your pathologist may see a large number of immune cells surrounding the cyst. Your pathologist will examine your tissue carefully to make sure there is no sign of cancer.

by Pavandeep Gill MD and Allison Osmond MD FRCPC (updated August 19, 2021)
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