In the rodent, the chorioallantoic placenta forms from the ectoplacental cone that differentiates into two regions that are morphologically distinguishable shortly after midgestation. These regions of the fetal placenta, termed the junctional and labyrinth zones, physically separate as the allantoic mesoderm invades the ectoplacental cone. Cells in the junctional zone contain the tropho-blast giant cells, spongiotrophoblast, and glycogen cells. As differentiation ensues, the junctional cells develop an invasive phenotype and become responsible for the production of placental hormones. The labyrinth zone is comprised of the trophoblast giant cells, syncytial trophoblast, and the fetal mesenchyme and vasculature. The labyrinth region comprises the site of fetal-maternal exchange and it is distinguished by the extent of penetration of the fetal vasculature.
In the present study, we have used RNA fingerprinting to search for candidate gene products that function in implantation and early placentation. Examination of the sequence from one of the cDNAs amplified from the implantation site identified the putative rat homolog of the mouse Hp58 gene, shown to be essential for embryogenesis, and yeast pep8, a gene involved in vacuole protein trafficking. Transgenic mice homozygous for the in-sertional mutation Hp58 were developmentally abnormal at Day 7.5 and exhibited irregularities in the amnion and chorion of the placenta. The allantois developed normally through Day 8.5, but it failed to fuse with the chorion to form the chorioallantoic placenta. In order to gain new insight into the function of Hp58 in the development and maturation of the chorioallantoic placenta, in this report we determined the temporal and spatial distribution of Hp58 in the early postimplantation uterus and fetal placenta. The temporal and spatial changes in Hp58 expression in these tissues are consistent with its performing multiple functions during placental development. In particular, the immuno-localization of Hp58 to the fetal erythroid cells during the emergence and maturation of the chorioallantoic placenta suggest that this protein may perform essential functions in the regulatory networks that control hematopoietic development.