Hair matrix differentiation. Occurrence in lesions other than pilomatricoma.

The formation of structures resembling hair matrices is the least common form of follicular differentiation and only occurs with any frequency in pilomatricoma. Rarely, follicular cysts and both benign and malignant adnexal tumors can show areas of pilomatricoma-like change, or hair matrix differentiation. Examples of focal hair matrix differentiation in a follicular cyst, a cutaneous mixed tumor, and an adnexal carcinoma are presented. Each of these cases also demonstrates the presence of limited differentiation toward structures of the inner root sheath.