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 immuno-histochemical analysis the tumor cells were immunoreactive with anti-S-100 protein and anticarcinoembryonic antigen, but they did not express cytokeratin or α-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 tubulove-sicular cristae. The presence of a well-developed, often multilayered basement membrane was confirmed at ultrastructural level. The activity of 3-β-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.