Trichilemmal cyst
human disease
A trichilemmal cyst (or pilar cyst) is a common cyst that forms from a hair follicle, most often on the scalp, and is smooth, mobile, and filled with keratin, a protein component found in hair, nails, skin, and horns. Trichilemmal cysts are clinically and histologically distinct from trichilemmal horns, hard tissue that is much rarer and not limited to the scalp. Rarely, these cysts may grow more extensively and form rapidly multiplying trichilemmal tumors, also called proliferating trichilemmal cysts, which are benign, but may grow aggressively at the cyst site. Very rarely, trichilemmal cysts can become cancerous.
Classification
Trichilemmal cysts may be classified as sebaceous cysts, although technically speaking are not sebaceous. "True" sebaceous cysts, which originate from sebaceous glands and which contain sebum, are relatively rare and are known as steatocystoma simplex or, if multiple, as steatocystoma multiplex. Medical professionals have suggested that the term "sebaceous cyst" be avoided since it can be misleading. In practice, however, the term is still often used for epidermoid and pilar cysts.
Pathogenesis
Trichilemmal cysts are derived from the outer root sheath of the hair follicle. Their origin is currently unknown, but they may be produced by budding from the external root sheath as a genetically determined structural aberration.
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