Spatial and temporal patterns of expression of messenger RNA for insulin-like growth factors and their binding proteins in the placenta of man and laboratory animals

To better understand the role of the insulin-like growth factors (IGF- I an d -II) and their binding proteins (IGFBPs 1-6) in placental development and function, it is important to review similarities and differences between s pecies in expression of the respective mRNAs. In human placenta, IGF-II mRN A is expressed in chorionic mesoderm and first trimester villous cytotropho blast, but not in syncytiotrophoblast. In contrast, in rhesus monkey placen ta, IGF-II mRNA is expressed in syncytiotrophoblast but not in chorionic me soderm. IGFBP-3 mRNA is present in the chorionic mesoderm of placental vill i from both these species and may modulate IGF-II action through a paracrin e mechanism. In rodent placentae, IGF-II mRNA is expressed both in fetal me soderm and in the trophoblast of the placental labyrinth. In guinea pig, wh ere IGFBP-5 mRNA is expressed in the marginal and interlobular syncytium an d IGF-PI mRNA in the labyrinth, interaction between IGF-II and IGFBP-5 mRNA may be involved in vascularization of the placenta by fetal vessels. In sh eep placenta, IGF-II mRNA is expressed, not in the trophoblast layer, but i n the fetal mesoderm immediately adjacent to it. In the basal plate of huma n, rhesus monkey and baboon placentae, extravillous trophoblasts express IG F-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 inter face of the primate placenta during trophoblast invasion and decidualizatio n. IGFBP-1 expressed by the decidua may also interact with alpha(5)beta(1) integrin expressed by the extravillous trophoblast. The placentae of rodent s are also of the invasive type. Glycogen cells of the mouse placenta are a nalogous with human extravillous trophoblast and express IGF-II mRNA. Howev er, expression of IGFBP mRNAs in the mouse, as in the guinea pig, is confin ed 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 tr ophoblasts and IGFBPs expressed in the vessel walls. However, it is possibl e that IGFBPs expressed by maternal vessels are associated with processes t hat are independent of trophoblast invasion. Thus, IGFBP-3 mRNA is highly e xpressed in the maternal blood vessels of the non-deciduate sheep placenta. Findings to date highlight the diversity in the expression of the IGF syst em among placentae of man and different laboratory animals, and even betwee n closely related species. Comparative studies will continue to be required to understand the functional role of IGFs and IGFBPs in each species. (C) 2000 Harcourt Publishers Ltd.