A pilomatrixoma is a hamartoma composed mainly of dead, calcified epithelial cells. It results from epithelial cells trying to differentiate into hair structures. Commonly, it occurs in childhood and to females. It has a rare association with myotonic dystrophy.
Clinically, it presents as a stony tumour of the face or arms. Typically several centimetres in size, it may be dermal or subcutaneous. Frequently, there is a clearly-delimiting capsule.
Histologically, the lesion matures by basophilic cells moving towards the centre of the lesion from the periphery. On route, 'shadow cells' form as they lose their nuclei and gain keratin. Immature hairs and other pilosebaceous structures may be seen. Calcification occurs in the shadow cells and the stroma.
Treatment is by excision.
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