The placenta plays a key role in pregnancy, mediating exchanges between mother and fetus and maternal tolerance of fetopaternal antigens. In some species, it also produces hormones that ensure the maintenance of gestation and fetal well-being. This unique organ also has considerable potential for use as a model for various aspects of biology. Indeed, the use of transgenic mouse models has greatly improved our understanding of the genetic control of placental development in this species and has opened up new ®elds of investigation in developmental biology. Analogous cell types have been identi®ed among human and murine trophoblasts: proliferative trophoblastic cells, invasive trophoblastic cells and cells differentiating into syncytium, but human and mouse placentas differ in both morphogenesis and endocrine function. Herein, the similarities and differences between the human and mouse models are reviewed, with a view to encouraging caution in the extrapolation of results from one model to the other.

We recently demonstrated that the product of the HERV-W env gene, a retroviral envelope protein also dubbed syncytin, is a highly fusogenic membrane glycoprotein inducing the formation of syncytia on interaction with the type D mammalian retrovirus receptor. In addition, the detection of HERV-W Env protein (Env-W) expression in placental tissue sections led us to propose a role for this fusogenic glycoprotein in placenta formation. To evaluate this hypothesis, we analyzed the involvement of Env-W in the differentiation of primary cultures of human villous cytotrophoblasts that spontaneously differentiate by cell fusion into syncytiotrophoblasts in vitro. First, we observed that HERV-W env mRNA and glycoprotein expression are colinear with primary cytotrophoblast differentiation and with expression of human chorionic gonadotropin (hCG), a marker of syncytiotrophoblast formation. Second, we observed that in vitro stimulation of trophoblast cell fusion and differentiation by cyclic AMP is also associated with a concomitant increase in HERV-W env and hCG mRNA and protein expression. Finally, by using specific antisense oligonucleotides, we demonstrated that inhibition of Env-W protein expression leads to a decrease of trophoblast fusion and differentiation, with the secretion of hCG in culture medium of antisense oligonucleotide-treated cells being decreased by fivefold. Taken together, these results strongly support a direct role for Env-W in human trophoblast cell fusion and differentiation.In humans, fetal cytotrophoblasts play a key role both in the embryo implantation process and in placental development. In early pregnancy, mononuclear cytotrophoblasts proliferate and invade the maternal endometrium to form the anchoring villi (4,14). Cytotrophoblasts also fuse and differentiate into a continuous layer of multinucleated syncytiotrophoblast. This cell layer, which covers the chorionic villi, is bathed by maternal blood in the intervillous spaces from early gestation (31, 34). The syncytiotrophoblast layer plays a major role throughout pregnancy, since it is the site of numerous placental functions including ion and nutrient exchange and the synthesis of steroid and peptide hormones required for fetal growth and development (13, 33). Some of these hormones, such as human chorionic gonadotropin (hCG) and human placental lactogen, are specific to pregnancy and can be used as markers of syncytium formation (20,22).It has been established both in vivo and in vitro that the syncytiotrophoblast layer arises from the differentiation and fusion of mononuclear cytotrophoblasts. Isolated mononuclear cytotrophoblasts aggregate and fuse to form a nonproliferative multinucleated syncytiotrophoblast which synthesizes and secretes specific hormones required for fetal development (2,25). This cytotrophoblast differentiation is stimulated in vitro by a number of factors such as growth factors (epidermal growth factor, granulocyte-macrophage colony-stimulating factor) and hormones (hCG, estradiol, and dexamethasone). We recently ...

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