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Myofibroblastic contraction in spontaneous regression of multiple congenital mesenchymal hamartomas

Subcutaneous nodules from a newborn boy with “multiple fibromatosis” involving the head, neck, trunk, and all four extremities were studied by light microscopy, transmission electron microscopy, and immunofluorescent techniques. Light microscopy suggested a hamartomatous process with fibroblastic, adipose, vasoformative and apparent smooth muscle components. The principal cell population combined ultrastructural characteristics of both fibroblasts and smooth muscle cells. Immunofluorescent studies revealed binding of human anti‐smooth muscle antibody to the cytoplasm of the spindle cell population of the subdermal nodules but not to fibroblasts of the overlying uninvolved skin. The ultrastructural and immunofluorescent studies revealed the previously undescribed fact that fibrous hamartoma of infancy is principally a proliferation of myofibroblasts. At age 8 months, there was complete spontaneous regression of all subcutaneous nodules not previously altered by excisional biopsy. The authors conclude that myofibroblasts are fibrocontractile cells, which play a role in shrinkage and eventual disappearance of these subdermal hamartomas.