Pilar cysts, also known as trichilemmal cysts, are one of the most common types of skin growths encountered by dermatologists. These generally benign, flesh-colored nodules develop beneath the skin’s surface and are a result of a specific cellular process within the hair follicle. Understanding the cause of these lumps involves looking closely at the structure of the hair and the role of genetics in their formation.
Anatomy and Appearance
A pilar cyst presents as a smooth, firm, and mobile lump located just beneath the skin. These growths are typically slow-growing and can vary in size from a few millimeters to several centimeters in diameter. They are almost always found on the scalp, with approximately 90% of all cases occurring in this location due to the high density of hair follicles there.
The cyst itself is a closed sac that originates from the outer root sheath of the hair follicle, which is the epithelial layer surrounding the hair shaft. Unlike other types of skin cysts, the wall of a pilar cyst lacks a granular cell layer, a feature that resembles the structure of the outer root sheath. Inside the sac, a thick, pasty material accumulates, which is composed of dense keratin and its breakdown products.
Genetic and Cellular Origin
The underlying cause of a pilar cyst involves a malfunction in the normal process of cell turnover within the hair follicle. The cells of the outer root sheath begin to proliferate rapidly. This excessive cellular activity leads to the creation of a closed sac, which then fills with the accumulating keratin and cellular debris.
The specific type of keratinization that occurs inside the cyst is a rapid, homogeneous process without the usual cell layers seen in the epidermis. This distinct mechanism is termed trichilemmal keratinization, which results in the compact, white or yellow filling of the cyst. The failure of these cells and keratin to shed normally creates pressure, causing the lump to push up against the outer skin.
A strong genetic component is a primary driver in the development of these cysts. Pilar cysts are often inherited in an autosomal dominant pattern, meaning a person needs only one copy of the affected gene from either parent to have a predisposition. This explains why family history is frequently observed. Children of an affected parent have a 50% chance of inheriting the trait, which is thought to be linked to a specific location on chromosome 3 in some hereditary cases.
When Treatment is Necessary
Pilar cysts are non-cancerous and generally pose no medical danger, making treatment unnecessary in most cases. A doctor may recommend simply monitoring a small, asymptomatic cyst. However, removal is often sought for cosmetic reasons, especially when the cyst is large or located in a noticeable area.
Removal becomes medically advisable if the cyst causes discomfort, pain, or becomes infected. A ruptured cyst can lead to an inflammatory reaction, causing tenderness, redness, and swelling. If infected, the cyst may require oral antibiotics to reduce inflammation before a surgical procedure can be performed.
The definitive treatment for a pilar cyst is complete surgical excision, where the entire cyst wall, or sac, is removed. Removing the sac is important because it prevents the cyst from recurring in the same spot by eliminating the source of the keratin production. A less invasive technique involves making a small incision to express the contents and then carefully removing the collapsed sac.