Trichilemmal cysts are surrounded by fibrous capsules against which rest layers of small, cuboidal, dark-staining basal epithelial cells in a palisade arrangement with no distinct intercellular bridging. These merge with characteristic squamous epithelium composed of swollen pale keratinocytes, which increase in height as they mature and transform abruptly into solid eosinophilic-staining keratin without the formation of a granular cell layer. They often have foci of calcification. [10] In contrast, epidermoid cysts have a granular cell layer in the lining epithelium. Epidermoid cysts (nonimplantation variant) have laminated keratin, which is believed to be derived from the follicular infundibulum. [10] Note the slides below.
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.
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.
Tricholemmal cyst keratin stains with antikeratin antibodies derived from human hair. In contrast, epidermal cyst keratin stains with human callus–derived antikeratin antibodies. [20]
Very rarely, sebaceous or apocrine differentiation occurs in a cyst wall. [21] Calcification may be seen, and a few reports have described ossification. [22, 23]
If cyst wall rupture occurs, a foreign body giant cell reaction may surround the cyst. [6, 10] Note the slides below.
Ruptured pilar cyst. A ruptured trichilemmal cyst showing replacement of the squamous lining by histiocytes and rare multinucleated giant cells.
Ruptured, low power. Another ruptured trichilemmal cyst showing replacement of the epithelial lining by granulomatous reaction with numerous cholesterol clefts and calcifications.
Ruptured, high power. Higher-power image highlighting the cholesterol clefts and calcifications.
Lobules of squamous epithelium in the cyst wall suggest a proliferating trichilemmal cyst. [1] Other histologic characteristics of proliferating trichilemmal cysts may include mitoses, cell atypia, and necrosis. Penetration of tumor cells through the cyst lining suggests malignancy. [3]
-
A firm, smooth swelling on the scalp.
-
Surgical removal of an intact pilar cyst through an elliptical excision.
-
Closure of the defect after surgical removal of the pilar cyst.
-
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.
-
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.
-
Numerous cholesterol clefts are identified within the homogenous keratin of the trichilemmal cyst.
-
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.
-
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.
-
Ruptured pilar cyst. A ruptured trichilemmal cyst showing replacement of the squamous lining by histiocytes and rare multinucleated giant cells.
-
Ruptured, low power. Another ruptured trichilemmal cyst showing replacement of the epithelial lining by granulomatous reaction with numerous cholesterol clefts and calcifications.
-
Ruptured, high power. Higher-power image highlighting the cholesterol clefts and calcifications.
