Embryonic and extra-embryonic lineages are separated at the blastocyst stage in the mouse at the onset of implantation but well ahead of implantation in most mammals. To provide information on the development of the trophoblast lineage in late-implanting bovine embryos, we combined the use of molecular markers defining embryonic and extra-embryonic lineages in the mouse with a transcriptomic approach dedicated to the early steps of the elongation process, a characteristic feature of blastocyst development in ruminants. In this study, we present molecular evidence for differences between the cow and the mouse in the programming of trophoblast differentiation. This different programming encompasses: (i) the expression of epiblast specifying genes (Oct-4, Nanog) in bovine trophoblast cells at the onset of elongation, (ii) the transcription of proliferation markers in early elongating blastocysts, (iii) the early detection of trophoblast-specific transcripts related to extra-embryonic tissue's differentiation (Hand1, Ets2, IFN-tau) and (iv) the identification of a new transcript (c12) which displays a reciprocal pattern to that of Oct-4 and Nanog genes in the embryonic cells and for which no equivalent has thus far been found in the mouse. Altogether, these results tended to show that early elongation is a critical transition in bovine trophoblast development.

A major unresolved issue in the cloning of mammals by somatic cell nuclear transfer (SCNT) is the mechanism by which the process fails after embryos are transferred to the uterus of recipients before or during the implantation window. We investigated this problem by using RNA sequencing (RNA-seq) to compare the transcriptomes in cattle conceptuses produced by SCNT and artificial insemination (AI) at day (d) 18 (preimplantation) and d 34 (postimplantation) of gestation. In addition, endometrium was profiled to identify the communication pathways that might be affected by the presence of a cloned conceptus, ultimately leading to mortality before or during the implantation window. At d 18, the effects on the transcriptome associated with SCNT were massive, involving more than 5,000 differentially expressed genes (DEGs). Among them are 121 genes that have embryonic lethal phenotypes in mice, cause defects in trophoblast and placental development, and/or affect conceptus survival in mice. In endometria at d 18, <0.4% of expressed genes were affected by the presence of a cloned conceptus, whereas at d 34, ∼36% and <0.7% of genes were differentially expressed in intercaruncular and caruncular tissues, respectively. Functional analysis of DEGs in placental and endometrial tissues suggests a major disruption of signaling between the cloned conceptus and the endometrium, particularly the intercaruncular tissue. Our results support a "bottleneck" model for cloned conceptus survival during the periimplantation period determined by gene expression levels in extraembryonic tissues and the endometrial response to altered signaling from clones. somatic cell nuclear transfer | conceptus | placentation | conceptus-maternal communication I n cattle, as in other mammals, exquisitely orchestrated physiological changes of the conceptus and uterus are necessary for a successful pregnancy. Synchronization of the complex events at the time of implantation relies on the timed release of molecular signals from the conceptus and the endometrium. Embryo-derived IFN-τ (IFNT) is the major signal of pregnancy in cattle, preventing luteolysis and regulating the expression of genes that are responsible for promoting local changes in the endometrium to accommodate the conceptus (1-3). In females, progesterone is the major driver of endometrial changes that prepare the uterus for conceptus implantation (4, 5). In addition to IFNT and progesterone, signaling between the bovine conceptus and the endometrium is bidirectional, and involves several pathways that work concomitantly (6) for the successful establishment of pregnancy.Independent studies have shown that the majority of embryonic losses in cattle occur during the period that spans embryo cleavage until the attachment of the blastocyst to the endometrium (7). The reasons for these losses remain unclear and likely result from several factors, including embryonic lethal genes (8, 9), environmental stressors (7), and endometrial condition (10). Cloning of cattle by somatic cell nuclear transfer ...