Summary Preimplantation embryos are vulnerable to the maternal environment. Exposure to environmental toxicants during this time may substantially damage the developmental program, leading to impaired fetal growth, birth defects, and life-long disease. Hence, there is a critical need to understand how the outcome of environmental exposures during early development affects embryogenesis and adult disease. The long-term goal of this research is to identify the mechanisms triggered by environmental exposure that disrupt the developmental programs. The objective of these studies is to determine whether the developmental toxicity of dioxins results from loss of pluripotency in the inner cell mass (ICM) of blastocysts. We propose the overarching hypothesis that the aryl hydrocarbon receptor (AHR) —the intracellular dioxin receptor—mediates the toxicity of dioxin by disrupting the regulation of the genes that control pluripotency during preimplantation embryogenesis. We believe that understanding how changes in ICM pluripotency are associated with genome- wide chromatin interactions will shed light on the mechanisms underlying the developmental toxicity of environmental exposures. We will address the following specific aims: (1) to determine whether dioxin exposure disrupts endogenous functions of the AHR in the regulation of ICM pluripotency; and, (2) to define changes in the pluripotency interactome resulting from deregulation of AHR signaling. These aims test the working hypothesis that dioxin exposure disrupts the endogenous interplay between the AHR and the core pluripotency factor OCT4 and modifies the OCT4 interactome, leading to loss of pluripotency in the ICM. In specific aim 1, the contribution to the ICM or to the trophoblast lineage will be determined for each one of the two totipotent 2-cell-stage blastomeres, subjected to low dose 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure. In specific aim 2, the OCT4 interactome will be investigated using HiChIP deep sequencing, a novel approach to study protein-centered three-dimensional (3D) chromatin interactions, in ICM cells of preimplantation blastocysts exposed to TCDD. We hope to develop an understanding of the fundamental mechanism(s) responsible for the developmental injuries caused by dioxin exposure. This work is innovative because it will use an advanced 3D chromatin structure approach to study toxic effects in pluripotency networks resulting from environmental exposure. Our in vivo model will lead to a better comprehension of the mechanisms of developmental toxicity of dioxins, making it possible to arrive at prevention and intervention approaches to deal with embryonic environmental injury. The expected outcome of this work is the establishment of a link between pluripotency loss and genome-wide chromatin interaction changes, which will provide the foundation to determine how these changes influence the in utero embryonic development.