Co-localization of placental lactogen-I, placental lactogen-II, and proliferin in the mouse placenta at midpregnancy.

This study was undertaken to determine whether mouse placental lactogen (mPL)-I, mPL-II, and proliferin (PLF) are expressed by the same population of placental giant cells at midpregnancy. Tissue sections from Day 9 of pregnancy were analyzed by double immunofluorescence staining. Sections were stained for PLF by use of a rhodamine-conjugated second antibody, and for mPL-I or mPL-II by use of a fluorescein-conjugated second antibody. All three proteins were present in most of the same giant cells. The distribution of mPL-I and PLF among giant cells in vitro was also examined. When placental cells from Day 7 of pregnancy were cultured for 5 days, > 90% of the cells that immunostained for mPL-I also immunostained for PLF on the first 3 days of culture. Thereafter, the percentage of cells that contained both proteins declined rapidly while the percentage that contained only PLF increased, suggesting continued differentiation of the cells in vitro. These data demonstrate that the same trophoblast giant cells express mPL-I, mPL-II, and PLF simultaneously at midpregnancy, suggesting that their gestational profiles in maternal blood during this period result at least partly from changes in gene expression in one population of cells and not from differentiation of several subsets of giant cells, each expressing only one member of the gene family.