Skin, Pilomatricoma

Pilomatricoma (also pilomatrixoma) is a benign lesion with differentiation toward the matrix of the hair follicle which accounts for almost 20% of pilar tumours. It is found particularly on the head, neck and upper extremities. About 60% develop in the first two decades of life. They are mostly solitary, but multiple lesions (usually less than five) are sometimes found. Some patients with multiple lesions have myotonic dystrophy. They have been reported in Turner’s syndrome, trisomy 9, Sotos syndrome, and with other abnormalities. A familial occurrence is rarely noted.

Pilomatricomas are firm nodules, approximately 0.5-3.0 cm in diameter. Pilomatricomas are typically slow growing, but rapid enlargement due to hemorrhage has been reported. Rarely, there is sufficient melanin pigment in the lesion to be visible clinically. These tumours have a variegated appearance macroscopically, with gray, white, and brown areas on the cut surface. Small spicules of bone and minute thorny fragments may be discernible. The consistency of the nodules depends on the amount of calcification and ossification.

There are two basic cell types, basophilic cells and eosinophilic ghost cells. The basophilic cells tend to be at the periphery of the cell islands and have little cytoplasm, indistinct cell borders, hyperchromatic nuclei, and plentiful mitoses. They may resemble the cells of a basal cell carcinoma. They are sometimes the predominant cell in lesions removed from elderly patients. The eosinophilic ghost cells have more cytoplasm and distinct cell borders, but no nuclear staining. These shadow cells form from the basophilic cells, and the transition may be relatively abrupt or take place over several layers of cells (transitional cells).

Calcification occurs in more than two-thirds of the tumours and is usually in the ghost cells. Ossification of the stroma occurs in about 13%, and melanin is present in nearly 20% of lesions and may be in the ghost cells as well as in the stroma. Bone morphogenetic protein (BMP-2), which plays an important role in ectopic bone formation, has been found in the ghost cells, suggesting that it may play a role in generating bone formation in pilomatrixomas.