Solitary form of infantile myofibromatosis: A histologic, immunohistochemical, and electronmicroscopic study of a regressing tumor over a 20-month period

We present the repeated clinical, histologic, immunohistochemical, and ultr astructural observations on a cutaneous myofibromatous tumor over a 20-mont h period. A 6-day-old Japanese female had a solitary tumor on her left wris t at birth. A biopsy was first performed at 16 days of age, when the tumor was likely fully developed. Thereafter, the tumor gradually regressed. A se cond biopsy was performed at 58 days of age, when the tumor was already in a phase of early regression. Finally, the tumor was resected at 20 months o f age, when it was in a phase of late regression. Our study demonstrated th at undifferentiated immature histiocytic cells predominated over spindle ce lls in the first biopsy specimen, but thereafter the former cells decreased or disappeared in parallel with the increase in the latter cells, which sh owed characteristics similar to myofibroblasts, in regressing lesions. This evidence suggests that the undifferentiated immature histiocytic cells are precursors of the spindle cells. Spindle cells in the phase of early regre ssion also showed many vacuoles and lipid-like droplets in the cytoplasm, e ven though they actively produced massive amounts of glycogen. These findin gs also suggest that tumor regression results from cytoplasmic vacuolation and disruption of spindle cells. Our results are considered to demonstrate, for the first time, the clinical and histologic features of the different developmental or regressive phases of infantile myofibromatosis.