Fibrous hamartoma of infancy: an immunohistochemical and ultrastructural study.

Fibrous hamartoma of infancy is an uncommon lesion of uncertain histogenesis. Three cases were studied by light microscopy, electron microscopy, and immunohistochemistry. Two histologic variants are presented. Ultrastructurally, fibroblasts and myofibroblasts were found in the fascicular-fibroblastic areas while primitive mesenchymal cells were found in the immature-appearing regions. Vimentin positivity was noted in both areas, whereas desmin and actin positivity was found mainly in the fascicular-fibroblastic regions. The lesion appears to fulfill the criteria for its characterization as "hamartoma."