Pathology of Ovarian Yolk Sac Tumors

Abstract

The human yolk sac tumor (YST) is still, in many ways, an enigmatic entity (Dehner 1989). Its identification as a tumor which reproduces yolk sac structures remains an excellent example of intellectual elaboration in the history of comparative pathology. However, its histogenesis and multi-potential capacity of differentiation continue to pose many questions. For although there is universal agreement on the germ cell nature of this neoplasm, evidence has been found in both experimental and human models of an origin in other types of cell. YSTs have been produced from placenta and displaced yolk sac remaining after fetectomy in the rodent (Sobis and Vandeputte 1977). In this experimental model (Sobis et al. 1989, 1991), yolk sac derived teratomas seem to arise from mature endodermal cells which are still capable of further differentiation into derivatives of all germ cell layers. Thus, these cells, despite their initial differentiation, are still capable of behaving like multipotential stem cells similar to germ cells. To explain this, a mechanism of transdiffer-entiation has been proposed (Sobis et al. 1991). Similarly, human YST may originate in somatic-type tissues of non-germ-cell origin, either in differentiated types such as those found in endometriosis (Lankerani et al. 1982), or in neoplastic ones, possibly by expressing a mechanism of neometaplasia or tumor heterogeneity (Rutgers et al. 1987). Furthermore, the formerly well delineated border that apparently existed between embryonal carcinoma (EC), YST, and seminoma has now become somewhat blurred (see Foreword), as human cell lines have been isolated with intermediate features of EC and YST (Vogelzang et al. 1985). It has been proposed (Takei and Pearl 1981) that EC could result from the proliferation of primitive endodermal cells of the YST that may be similar to the primordial germ cells recapitulating the germ cell formation by the human yolk sac. Finally, there is also a large percentage of ECs that have YST elements, making it difficult to distinguish each type of cell and pattern. The present YST terminology reflects only the extraembryonal parietal or visceral yolk sac differentiation and may thus cause confusion since many other types of somatic cells and tissues are known to differentiate in these tumors, such as intestinal and hepatic (Salazar et al. 1974; Nogales et al. 1978; Prat et al. 1982), mesenchymal (Michael et al. 1989), and perhaps even pulmonary primitive structures, since, in our view, some YSTs closely resemble the epithelial type of pulmonary blastoma (Yousem et al. 1990). It would seem that YSTs are capable of differentiating the full potential of the endoderm, i.e., the primitive gut with all its derivatives as well as the mesenchyme, as it is known that the early endoderm gives rise to the mesoblastic reticulum (Luckett 1978) and differentiates into the matrix-producing cells of the extraembryonic mesoderm (Enders and King 1988, see Chap. 2). On the fringe of endodermal differentiations there are some tumor entities that have many points in common with YST and may be in some way related to them. Among these is the small cell carcinoma occurring in young women, which has many clinical and ultra-structural similarities with YST (Ulbright et al. 1987), and certain types of mucinous cystadenoma showing AFP and α₁-antitrypsin-positive hyaline globules, which may be indicative of a full endodermal (gut) differentiation (Nogales 1987).