Ultrastructure and immunohistochemistry of a fetal-type Leydig cell tumor

A symptomless scrotal mass was removed from a 34-year-old man. The lesion was 7 cm in diameter and it was grossly a hemorrhagic cyst with indurated walls. By light microscopy tumor cell clusters and cords were seen infiltrating the testicle, tunica albuginea, and paratesticular tissue. In the immunohistochemical analysis the tumor cells were immunoreactive with anti-S-100 protein and anticarcinoembryonic antigen, but they did not express cytokeratin or alpha-fetoprotein as tested with paraffin sections. Tumor cell clusters were enveloped by a laminin-positive basement membrane. Electron microscopy revealed abundant smooth endoplasmic reticulum, lipid droplets, and membranous whorls in the cytoplasm. Lamellar whorled bodies were also seen in mitochondria, which contained tubulovesicular cristae. The presence of a well-developed, often multilayered basement membrane was confirmed at ultrastructural level. The activity of 3-beta-hydroxysteroid dehydrogenase suggested that the tumor cells were capable of androgen synthesis. The morphological features are reminiscent of fetal-type Leydig cells and are distinctly different from the Leydig cell tumors described so far.