Differentiation of the extra-embryonic trophoblasts is the essential first step for implantation of the mammalian embryo. Trophoblasts connect the maternal and embryonic compartments by attaching to uterine decidual cells, invade maternal blood vessels and contact maternal blood, and secrete hormones that regulate maternal and fetal physiology. In rodents, the trophoblasts that form the perimeter of the embryonic compartment at the implantation site are called giant cells. These cells arise through an abrogation of the normal cell cycle and the establishment of a program of repeated rounds of DNA synthesis without cell division. The transformation from a diploid trophoblast to a giant cell is hypothesized to be induced by factors that also regulate trophoblast giant cell-specific gene expression. Two such transcription factors are GATA-2 and GATA-3. Experiments are proposed to test the possibility that GATA-2 and GATA-3 are determinants in the switch from proliferative trophoblasts to terminally differentiated giant cells in culture, and regulators of placental development in vivo. Trophoblast cells will be manipulated in culture and in vivo to increase or decrease GATA factor activity, and the consequence of these changes on cell differentiation and development will be examined. Furthermore, although GATA-2 and GATA- 3 are expressed in a restricted set of cell types, they are not expressed exclusively in trophoblast giant cells. Thus, giant cell- specific gene expression, which depends on GATA factor activity, must also depend on other transcriptional regulatory proteins. Three factors, AP-1, Hxt, and p53, will be tested as components along with GATA-2 and GATA-3 of a combinatorial code for giant cell-specific gene expression. Finally, giant cells undergo further differentiation during gestation as revealed by the spectrum of hormones they secrete. The mechanism of this mid-to late-gestation differentiation is likely to depend on changes in the transcriptional activators or repressors present in giant cells, so the elements and factors responsible for mid/late compared to early/mid pregnancy giant cell gene expression will be analyzed.