Hair Sheath Keratins in Follicular Tumors: Identification in Keratoacanthoma and Their Significance as Differential Criteria from Squamous Cell Carcinoma

The study identified hair sheath keratins in keratoacanthoma (KA) tumors, distinguishing them from squamous cell carcinoma (SCC). Keratins were extracted from 3 KA tumors, revealing hair sheath keratins in all and epidermal keratin in only 1. This indicated that KA was a follicular tumor. A re-examination of 17 KA cases over 13 years showed significant histopathological features such as anagen keratinocytes, squamous eddies, trichilemmal keratinization, and clear cells. These features, particularly the presence of trichilemmal keratinization and clear cells forming small whirl structures, were observed in nearly all tumors. The study concluded that examining follicular structures could serve as an important criterion for differentiating KA from SCC histopathologically.