The pluripotential nature of the mesenchyme-like component of yolk sac tumor.

Germ cell neoplasms were reviewed for the investigation of a mesenchyme-like component of yolk sac tumor (YST) characterized by spindle cells with few mitoses in a myxoid, vascular background. Nineteen YSTs with this pattern were identified. The mesenchyme-like component of these YSTs appeared to derive from the epithelial elements of YST, since cytokeratin as well as vimentin positivity occurred in the spindle cells of the mesenchyme-like areas and foci of epithelial-spindle cell transition were present. In some cases the mesenchyme-like component showed differentiated mesenchymal elements (usually skeletal muscle). Similar features were identified in 13 chemotherapy-treated cases of YST that consisted only of this mesenchyme-like component. The mesenchyme-like component of YST appears to represent a chemoresistant, pluripotential cell population arising from metaplasia of YST epithelium; it may give rise to sarcomas occurring in some patients with treated germ cell tumors.