Roles of cytokines and growth factors in embryo-uterine signaling during the onset of implantation have been derived from studies in the mouse and rat, in which ovarian estrogen is required to initiate implantation. However, it is unknown whether the same signaling systems are operative in species in which implantation can occur in the progesterone (P4) primed uterus in the absence of estrogen. The applicant will address this question using hamsters as an animal model because like rabbits, guinea pigs, monkeys and perhaps humans, hamsters require only P4, but not estrogen, for establishment of uterine receptivity and implantation. However, hamsters share some common features of implantation, such as increased capillary permeability and stromal decidualization at the site of implantation. As a starting point, the applicant will focus on two molecules, leukemia inhibitory factor (LIF) and heparin binding-EGF-like factor (HB-EGF) that are important for implantation in mice. The applicant's preliminary results suggest that LIF, HB-EGF, and their receptors are expressed in the pregnant hamster uterus. Thus, the working hypothesis to be tested in this proposal is that uterine expressed LIF and HB-EGF interact with their complementary receptors in executing embryo-uterine interactions in paracrine/juxtracrine manners during implantation in hamsters, and these interactions occur under the influence of P4 alone. The applicant's specific aims are to study in hamsters: 1) the spatiotemporal expression of LIF, HB-EGF and their receptors, as well as ligand-induced phosphorylation of these receptors (erbBl and erbB4, LIF-R and gp 130) in the uterus on days 1-4 (preimplantation and implantation) and days 5-6 (postimplantation) in normal control and in P4-treated hypophysectomized pregnant hamsters; 2) the separate and interactive effects of estradiol-17beta (E2) and/or P4 on uterine expression of LIF, HB-EGF and their receptors in hypophysectomized hamsters, and evaluation of normal """"""""window of receptivity"""""""" for implantation as well as hormonal requirements for uterine receptivity and non-receptivity; and 3) the expression of LIF, HB-EGF and their receptors in the preimplantation embryo and evaluation of ligand-receptor signaling with LIF and HB-EGF in embryo- uterine interactions during implantation. To accomplish these aims, the applicant will use RT-PCR, Northern and in situ hybridization, immunocytochemistry, cross-linking, receptor phosphorylation, embryo culture and transfer studies. It is anticipated that with better understanding of the mechanisms of the development of uterine receptivity and embryo-uterine interactions for implantation, fertility and infertility in women could be more effectively managed.