Leiomyoblastoma of the uterus: an immunohistochemical and electron microscopic study of distinctive tumours with immature smooth muscle cell differentiation mimicking fetal uterine myocytes

Aims: We present three distinctive uterine tumours which exhibited immature smooth muscle differentiation mimicking smooth muscle cells of the fetal uterus. Methods and results: The patients were 45, 46 and 49 years old, and all of them had simple hysterectomies. Grossly, all tumours were present in the uterine body, and two of the three tumours were well demarcated 60-mm and 85-mm lesions, and the other tumour was a small 25-mm incidental lesion within multiple conventional leiomyomas. The tumours had varied histological features and were composed of round epithelioid, rhabdoid and large vacuolated cells intermingled with spindle-shaped cells to various degrees. Although their round vesicular nuclei showed mild to moderate variation in size, prominent nuclear atypia was not seen. Necrosis and mitotic figures suggesting biological aggressiveness were not present in any of the tumours. Immunohistochemically, tumour cells were intensely positive for desmin and alpha-smooth muscle actin, whereas positivity for heavy molecular weight caldesmon was restricted. In addition, two cases were positive for non-muscle myosin heavy chain (SMemb). Ultrastructurally, most tumour cells contained various amounts of intermediate filaments which were occasionally abundant and aggregated as in rhabdoid cells. Well-developed myofilaments with focal densities were observed in only a few tumour cells. Intermediate filaments and bundles of thin filaments without dense bodies were often intermingled and they occasionally formed distinctive complexes with many irregular dense body-like structures and crystalloid bodies. Other cytoplasmic organelles including rather rich mitochondria, some rough endoplasmic reticulum and free ribosomes were also common. Conclusions: These findings support their immature smooth muscle cell differentiation which mimics the mesenchymal cells of fetal uterus during 14-26 weeks of gestation. The term 'uterine leiomyoblastoma' is thought to be appropriate for describing these distinctive immature smooth muscle tumours.