Mesenchymally-derived Insulin-like growth factor 1 provides a paracrine stimulus for trophoblast migration
To better understand the role of the insulin-like growth factors (IGF-I and -II) and their binding proteins (IGFBPs 1-6) in placental development and function, it is important to review similarities and differences between species in expression of the respective mRNAs. In human placenta, IGF-II mRNA is expressed in chorionic mesoderm and first trimester villous cytotrophoblast, but not in syncytiotrophoblast. In contrast, in rhesus monkey placenta, IGF-II mRNA is expressed in syncytiotrophoblast but not in chorionic mesoderm. IGFBP-3 mRNA is present in the chorionic mesoderm of placental villi from both these species and may modulate IGF-II action through a paracrine mechanism. In rodent placentae, IGF-II mRNA is expressed both in fetal mesoderm and in the trophoblast of the placental labyrinth. In guinea pig, where IGFBP-5 mRNA is expressed in the marginal and interlobular syncytium and IGF-II mRNA in the labyrinth, interaction between IGF-II and IGFBP-5 mRNA may be involved in vascularization of the placenta by fetal vessels. In sheep placenta, IGF-II mRNA is expressed, not in the trophoblast layer, but in the fetal mesoderm immediately adjacent to it. In the basal plate of human, rhesus monkey and baboon placentae, extravillous trophoblasts express IGF-II mRNA and uterine decidual cells IGFBP 1-6 mRNAs. The inference is that there is interaction between IGF-II and IGFBPs at the maternal-fetal interface of the primate placenta during trophoblast invasion and decidualization. IGFBP-1 expressed by the decidua may also interact with alpha(5)beta(1)integrin expressed by the extravillous trophoblast. The placentae of rodents are also of the invasive type. Glycogen cells of the mouse placenta are analogous with human extravillous trophoblast and express IGF-II mRNA. However, expression of IGFBP mRNAs in the mouse, as in the guinea pig, is confined to non-decidualized endometrium and myometrium. IGF-II mRNA is strongly expressed by trophoblasts invading uterine vessels in human and guinea pig placentae. Interactions probably occur between IGF-II expressed by these trophoblasts and IGFBPs expressed in the vessel walls. However, it is possible that IGFBPs expressed by maternal vessels are associated with processes that are independent of trophoblast invasion. Thus, IGFBP-3 mRNA is highly expressed in the maternal blood vessels of the non-deciduate sheep placenta. Findings to date highlight the diversity in the expression of the IGF system among placentae of man and different laboratory animals, and even between closely related species. Comparative studies will continue to be required to understand the functional role of IGFs and IGFBPs in each species.