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polyembryoma

Monday 21 February 2011

The polyembryoma is a distinctive form of mixed germ cell tumor of the gonads that is often separately categorized because of its unique histologic features.

First described by Peyron, its formation of embryo-like structures composed of primitive germ cell tumor components may be viewed conceptually as a bridge between the primitive and differentiated (teratoma) germ cell tumor types, leading some to place the polyembryoma into the category of the most immature of all teratomas.

Indeed, perhaps every tumor of this type encountered to date has had at least a few differentiated teratomatous structures, such as glands, buttressing this viewpoint.

The tumor consists of small embryo-like bodies having a central ’germ disc’ composed of embryonal carcinoma epithelium and two cavities, a dorsal one recapitulating the amniotic cavity and a ventral one recapitulating the yolk sac cavity.

The latter is separated from the germ disc by a thin layer of yolk sac epithelium. The embryoid bodies lie in an edematous to myxoid stroma that has prominent blood vessels.

They are usually somewhat evenly spaced and, as noted earlier, the intervening areas show minor degrees of glands or other more differentiated elements.

Elements other than the embryoid bodies, however, should constitute less than 10% of the tumor for the ’polyembryoma’ designation to be used. Hepatoid cells may rarely be seen, and syncytiotrophoblast giant cells are more frequent, sometimes causing endocrine manifestations.

In some cases, the embryonal carcinoma and yolk sac epithelium proliferate to a limited degree and lose the orderly arrangement of the ’classic’ embryoid body, with extension of these components beyond the confines of the embryoid body.

The point at which one considers a small aggregate of cells a separate focus of embryonal carcinoma, or alternatively, yolk sac tumor is arbitrary, with one proposal being 3 mm.

Just as the presence of differentiated structures links the polyembryoma with the teratomas, so at the other end of the spectrum the phenomenon just referred to (’fragmentation of embryoid bodies’), links them with the primitive germ cell tumors.

Further example of this linkage is provided by the fact that if one looks carefully embryoid bodies are common in mixed germ cell tumors of the gonads and, simply because mixed germ cell tumors are more common in the testis and have more randomly arranged elements, embryoid bodies are typically a finding in testicular tumor pathology to a much greater degree than in the ovary.

A further distinctive arrangement of yolk sac tumor and embryonal carcinoma epithelium is seen in the pattern that has been likened to a necklace and referred to as ’diffuse embryoma’.

Like the polyembryoma, pure examples of the diffuse embryoma are exceptionally rare, although this is a relatively frequent, focal pattern in mixed germ cell tumors of the testis.

See also

- germ cell tumors