Proliferin (PLF), a member of the PRL/GH family secreted by the placenta, can be detected in both the maternal and fetal compartments. We now show that PLF immunoreactivity can be detected in association with the yolk sac, consistent with the transport of PLF across this structure into the amniotic fluid. Furthermore, PLF is transported across the extraembryonic membranes in isolated conceptuses that are placed in culture, and specific binding sites for PLF are detected in these embryos. The major binding sites for PLF in the cultured conceptus correspond to sites at which endogenous PLF localizes in the fetus, including developing vertebral and vascular structures. Similar binding patterns were also detected for PLF that was incubated with fetal sections. Competition and comparative binding studies indicate that the insulin-like growth factor II/mannose 6-phosphate receptor is involved in PLF binding to specific cells in the fetus. These results suggest that in addition to the effects of PLF in the placenta on neovascularization and in the maternal uterus on cell proliferation, PLF may also act at specific sites in the developing fetus.
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