Pilar or trichilemmal cysts are clinically indistinguishable from epidermal cysts but differ from them in frequency and distribution. They are less common than epidermal cysts, constituting only about 25% of the combined material; about 90% occur on the scalp . Pilar cysts often show an autosomal dominant inheritance pattern and are solitary in only 30% of the cases, with 10% of patients having more than 10 cysts . Furthermore, in contrast to epidermal cysts, pilar cysts are easily enucleated and appear as firm, smooth, white-walled cysts .
|
The epithelial cells close to the cystic cavity appear swollen and are filled with pale cytoplasm . These swollen cells do not produce a granular layer but generally undergo abrupt keratinization, although nuclear remnants are occasionally retained in a few cells. The content of the cysts consists of homogeneous eosinophilic material .
|
Whereas focal calcification of the cyst content does not occur in epidermal cysts, foci of calcification are seen in approximately one fourth of trichilemmal cysts . A considerable foreign-body reaction results when the wall of a trichilemmal cyst ruptures, and the cyst may then undergo partial or complete disintegration.
|
Trichilemmal cysts frequently disclose small, acanthotic foci in their walls that are indistinguishable from solid areas, as seen in a proliferating trichilemmal cyst . The association of a trichilemmal cyst with tumor lobules of a proliferating trichilemmal cyst is also seen occasionally .
|
Pathogenesis.
Pilar cysts were originally called sebaceous cysts. The name was changed when it became apparent that the keratinization in them is analogous to the keratinization that takes place in the outer root sheath of the hair, or trichilemma . The outer root sheath of the hair does not keratinize wherever it covers the inner root sheath. It keratinizes normally in two areas-the follicular isthmus of anagen hairs and the sac surrounding catagen and telogen hairs-because in these two regions the inner root sheath has disappeared. The follicular isthmus of anagen hairs is the short, middle portion of the hair follicle, extending upward from the attachment of the arrector pili muscle to the entrance of the sebaceous duct. At the lower end of the follicular isthmus, the inner root sheath sloughs off, exposing the outer root sheath, which, in its exposed portion, undergoes a specific type of homogeneous keratinization without the interposition of a granular layer. This type of trichilemmal keratinization also takes place in the sac surrounding catagen and telogen hairs because hairs in these stages have lost their inner root sheath. The differentiation toward hair keratin in trichilemmal cysts has been confirmed by immunohistochemical staining because they stain with antikeratin antibodies derived from human hair, in contrast to epidermal cysts, which stain with anti keratin antibodies obtained from human callus .
|
Electron microscopic examination of the epithelial lining of trichilemmal cysts shows that, on their way from the peripheral layer toward the center, the epithelial cells have an increasing number of filaments in their cytoplasm. The transition from nucleate to anucleate cells is abrupt and is associated with the loss of all cytoplasmic organelles. The junction between the keratinizing and keratinized cells shows interdigitations . The keratinized cells are filled with tonofilaments and, unlike those in epidermal cysts, retain their desmosomal connections .
|
Differential Diagnosis. Even though both the pilar cyst and the proliferating trichilemmal cyst show trichilemmal types of keratinization and can occur together, one is essentially a cyst and the other essentially a solid, tumorlike proliferation.
|
|