Cutaneous myxoid fibroblastoma. A histological, immunohistochemical, and ultrastructural study.

A fibroblastic skin tumor with a myxoid matrix is reported that cannot be easily classified as one of the well-known entities of fibrous/fibrohistiocytic and myxoid skin tumors. A 27-year-old white woman presented with a reddish, dome-shaped cutaneous nodule 8 mm in diameter on the left popliteal fossa that had developed spontaneously within the preceding 2 years. There was no sign of recurrence 30 months after excision. Light microscopic examination showed a well-circumscribed tumor confined to the upper dermis and consisting of stellate and spindle-shaped cells arranged loosely in a fascicular pattern resembling tissue cultures of fibroblasts. There were almost no collagen bundles between tumor cells, and Mowry's staining showed large amounts of glycosaminoglycans. Immunohistochemical studies of the tumor cells showed reactivity only to vimentin, whereas markers of histiocytes, dermal dendrocytes, and neurogenic and myogenic differentiation were negative. By electron microscopy, the majority of tumor cells contained elliptical nuclei, but some tumor cells had conspicuous multisegmented nuclei with several large and small nuclear segments connected by thin nuclear bridges (labyrinth nuclei). Single fibrils were found within the interstitium; collagen fibers were rare. Histological and ultrastructural examinations identified tumor cells as fibroblasts. High cellularity distinguishes this tumor from cutaneous myxoma. We conclude that this lesion represents a newly recognized tumor of fibroblastic origin. The name cutaneous myxoid fibroblastoma is proposed.