Cellular localization of the human chorionic gonadotropin beta-subunit in transgenic mouse placenta

CG is a human placental glycoprotein expressed in first-trimester trophobla sts. To examine the regulation of the CGP-subunit in an in vivo model, we p reviously constructed transgenic mice containing the CGP gene cluster and d emonstrated its expression in the placenta. Here, we determine the cell typ e responsible for CGP synthesis in the mouse by immunohistochemical and in situ hybridization analyses. Unexpectedly, the protein and messenger RNA we re not detected in trophoblast or elsewhere in the chorioallantoic placenta but in the parietal endoderm, a separate extraembryonic component of the p lacenta. The identity of this CGP-producing layer was confirmed by the pres ence of laminin A, a known protein of the parietal endoderm extracellular m atrix. However, we observed heterogeneity, with respect to synthesis of lam inin A and CGP; parietal endoderm cells expressing CGP at high levels synth esized less laminin A, and vice versa. The absence of CGP production in tro phoblasts of the transgenic mouse demonstrates a lack of transcriptional eq uivalence between rodent and human trophoblasts. The data are consistent wi th the hypothesis that, in human placenta, one or more transcriptional fact ors coevolved as members of the CGP gene cluster underwent duplication.