Gonadal endodermal sinus (yolk sac) tumor with pure intestinal differentiation: a new histologic type.

We studied 3 cases of a variant of endodermal sinus tumor (EST) or yolk sac tumor (YST). Two tumors originated in the ovary and one in the testis. The patients had very high levels of serum alpha-fetoprotein (AFP). All three tumors had a characteristic histologic appearance and were composed of acinar structures lined by columnar epithelium with large, immature nuclei. Immunohistochemistry and electron microscopy, including freeze-fracture studies, confirmed that this unusual tumor is one with exclusive intestinal differentiation. We feel that this unique tumor is a pure EST (YST) with exclusive intestinal differentiation.