Background
Trichilemmal or pilar cysts are common intradermal or subcutaneous cysts, occurring in 5-10% of the population. [1, 2] Greater than 90% occur on the scalp, where trichilemmal cysts are the most common cutaneous cyst. [3, 4] Trichilemmal cysts are almost always benign. They may be sporadic or they may be autosomal dominantly inherited. [5, 6] They contain keratin and its breakdown products and are lined by walls resembling the external (outer) root sheath of the hair.
In 2% of trichilemmal cysts, single or multiple foci of proliferating cells lead to tumors called proliferating trichilemmal cysts. [7] Proliferating trichilemmal cysts are gradually enlarging (up to 25 cm in diameter), exophytic nodules that occasionally ulcerate. [3] Although biologically benign tumors, they may be locally aggressive. Recurrences and metastases have been observed, with rare malignant transformation. [6, 7, 8, 9]
Pathophysiology
Erroneously but often called sebaceous cysts, trichilemmal cysts are lined by stratified squamous epithelium similar to that in the isthmus of the hair follicle. [6] This is the segment between the insertion of the erector pili muscle and the sebaceous gland duct, where no inner root sheath exists. The keratinization is similar to that which occurs in the outer root sheath. [10] The squamous epithelium undergoes “trichilemmal keratinization” or rapid keratin formation without a granular cell layer, resulting in a cyst wall without a granular cell layer. [3, 10]
Etiology
If the cysts are hereditary, the inheritance pattern is usually autosomal dominant. [5, 6] Hereditary trichilemmal cysts link to the short arm of chromosome 3. [11]
Epidemiology
US frequency
Trichilemmal cysts are common, occurring in 5-10% of the population. [1]
Race
Trichilemmal cysts have no known racial predilection.
Sex
Trichilemmal cysts occur more commonly in women than in men. [1]
Age
Trichilemmal cysts are more common in persons of middle age than in younger persons. [1]
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A firm, smooth swelling on the scalp.
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Surgical removal of an intact pilar cyst through an elliptical excision.
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Closure of the defect after surgical removal of the pilar cyst.
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The trichilemmal cyst is lined by a squamous epithelium without a granular layer and with swelling of the cells close to the cyst cavity, which is filled with homogenous keratin.
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At higher magnification, the cyst is lined by a squamous epithelium without a granular layer and with swelling of the cells close to the cyst cavity, which is filled with homogenous keratin.
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Numerous cholesterol clefts are identified within the homogenous keratin of the trichilemmal cyst.
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Pilar cyst, low power. The trichilemmal cyst is lined by squamous epithelium without a granular layer (trichilemmal keratinization) and with swelling of the cells close to the cyst cavity, which is filled with homogenous keratin. Calcifications are a common feature.
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Pilar cyst, medium magnification. Higher magnification shows the stratified squamous epithelium without a granular layer and shows swelling of the cells closest to the cyst cavity.
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Ruptured pilar cyst. A ruptured trichilemmal cyst showing replacement of the squamous lining by histiocytes and rare multinucleated giant cells.
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Ruptured, low power. Another ruptured trichilemmal cyst showing replacement of the epithelial lining by granulomatous reaction with numerous cholesterol clefts and calcifications.
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Ruptured, high power. Higher-power image highlighting the cholesterol clefts and calcifications.
