Psx homeobox gene is X-linked and specifically expressed in trophoblast cells of mouse placenta.

We previously isolated a cDNA clone for a homeobox-containing gene with its expression restricted to the extraembryonic tissues. In this study, Psx gene expression was further examined using in situ hybridization to determine the cellular distribution of Psx transcripts during embryo development. Psx expression was first detected at embryonic day 8.5 only in trophoblast giant cells and chorionic ectoderm. At E 9.5 and E 13.5, the expression was restricted to the giant cells and the labyrinthine trophoblast layer. In addition, the gene expression was detected in differentiated Rcho-1 trophoblast cells in vitro, which is typical of trophoblast giant cells in vivo, but not in proliferating Rcho-1 cells and HRP-1 cells. Interestingly, rat Psx homologue mRNA is about 200 bp shorter than mouse Psx, suggesting that there is a high degree of sequence divergence between the mouse and rat Psx homologues. The sequence divergence, perhaps as a result of rapid evolution, is further supported by the zoo blot analysis because the Psx gene was detectable only in mouse and rat but not in other vertebrate species tested. Psx is localized to the murine X chromosome. Taken together, our results suggest that Psx gene plays a unique role in the function of differentiated trophoblast cells and also serves as a useful model for studying trophoblast cell lineages and the rapid evolution of homeobox genes.