Trichilemmal cyst - Skin -

This article will help you read and understand your pathology report for trichilemmal cyst.

by Pavandeep Gill, MD and Allison Osmond, MD FRCPC, updated on May 14, 2019

Quick facts:

  • A trichilemmal cyst is a non-cancerous growth that starts in a hair follicle.
  • It is a very common condition and most start on the scalp.
  • Another name for trichilemmal cyst is a pilar cyst.

The anatomy of the skin

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.

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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 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.

What is a trichilemmal cyst?

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

Another name for a trichilemmal cyst is a pilar cyst.

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

Trichilemmal 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.

The diagnosis of trichilemmal 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.

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