We have shown that the neu oncogene product (p185(neu)) is not present
in the rat embryo before organogenesis. However, coincident with the
onset of organogenesis, p185(neu) was detected in neural and connectiv
e tissue as well as in the secretory epithelium as was described by Ko
kai et al. (1987). In addition, p185(neu) is also expressed in the rat
visceral yolk sac (VYS) endodermal cells but not in the mesenchymal a
nd mesothelial layers of the same structure nor in the amnion. The fir
st detectable sign of p185(neu) expression in VYS was found at d 11 of
gestation and the levels of protein increased towards the end of preg
nancy. In the yolk sac carcinoma (YSC), which is considered to be the
malignant counterpart of the rat yolk sac, p185(neu) was observed only
within columnar epithelial cells (the visceral component of the neopl
asm) while parietal endoderm-like cells were devoid of detectable prot
ein. From d 9 of pregnancy up to delivery some of the trophoblastic gi
ant cells also showed a faint to moderate immunoreactivity. Results ar
e presented which would indicate a possible role of p185(neu) in rat e
mbryogenesis.