Primary osteoma cutis. Clinical, morphological, and ultrastructural study.

Primary osteoma cutis arises in the deeper dermis for no apparent reason and presents as mature, lamellar, and osteonic bone; secondary cutaneous osteomas are correlated with inflammatory processes, scars, or dysembryoplasia and are always composed of osteoid. Ultrastructural findings of primary cutaneous osteomas have not been reported to date. Light and electron microscopic findings of a case of primary osteoma cutis are described: mineralized areas may be divided into macrocalcification and microcalcification. Macrocalcification consists of lamellar bone. Osteocytes populate the lamellae, whereas collagen fibril distribution is bone-like. Hydroxyapatite deposition presents as globular or needle-like electron-dense material progressively masking the connective tissue matrix. Microcalcifications, which are found in macroscopically normal dermis around the calcified plaque, consist of osteoid tissue inhabited by osteoblast-like cells. Microcalcifications may be interpreted as metastatic calcifications related to the primary osteoma calcified plaque. Primary osteoma cutis may be considered as true bone amartothic formation rather than dermal mineralization.