Ossifying trichilemmal cyst.

Although a trichilemmal cyst is a relatively common benign cutaneous condition and ossification has been observed within many cutaneous lesions to our knowledge, the association between ossification and trichilemmal cyst has not been previously reported. We describe for the first time the case of a trichilemmal cyst with marked osseous metaplasia arising on the scalp of a 46 year-old woman. Microscopically the lesion showed the typical features of a trichilemmal cyst with the finding of intra-extraluminal foci of calcification and the exceptional feature of mature bone formation. No areas of mature cartilage were observed near the focus of ossification. The cutaneous bone may have been directly formed from osteogenic stromal elements without a cartilaginous precursor (membranous or mesenchymal ossification). The dystrophic calcification might also contribute to the bone-forming process.