*3.4. Yolk Sac Cells*

A very limited number of UC with yolk sac tumor differentiation has been reported in the literature [52–55]. The yolk sac differentiation represents an example of a somatic differentiation present in non-gonadal neoplasms [56]. A varied spectrum of patterns have been identified in these tumors: microcystic, vitelline, glandular enteric-like, hepatoid, solid, sarcomatoid, etc. An enteroblastic differentiation seems to be the most frequent histology in somatically derived yolk sac tumors [56].

Immunohistochemistry is useful to identify yolk sac differentiation in UC and other somatic tumors considering the wide spectrum of patterns that can be detected in this tumor. Alpha-fetoprotein and SALL4 are consistently positive. CK7, however, is negative. Markers of intestinal differentiation, like CDX2, are usually positive in enteroblastic areas and Her Par-1 in hepatoid ones. A polysomic abnormality in 12p has been detected in one recently published case [55].
