p185neu is expressed in yolk sac during rat postimplantation development.

We have shown that the neu oncogene product (p185neu) is not present in the rat embryo before organogenesis. However, coincident with the onset of organogenesis, p185neu was detected in neural and connective tissue as well as in the secretory epithelium as was described by Kokai et al. (1987). In addition, p185neu is also expressed in the rat visceral yolk sac (VYS) endodermal cells but not in the mesenchymal and mesothelial layers of the same structure nor in the amnion. The first detectable sign of p185neu expression in VYS was found at d 11 of gestation and the levels of protein increased towards the end of pregnancy. In the yolk sac carcinoma (YSC), which is considered to be the malignant counterpart of the rat yolk sac, p185neu was observed only within columnar epithelial cells (the visceral component of the neoplasm) while parietal endoderm-like cells were devoid of detectable protein. From d 9 of pregnancy up to delivery some of the trophoblastic giant cells also showed a faint to moderate immunoreactivity. Results are presented which would indicate a possible role of p185neu in rat embryogenesis.