Numerous studies in many animals including livestock species such as cattle have demonstrated the detrimental effects of culturing gametes and embryos in non-physiological culture systems and are considered valuable models for developing reproductive biotechnologies and evaluating the effects of ART in humans. In vitro culture promotes excessive reactive oxygen species production that can override an embryo’s antioxidant defenses producing oxidative stress that triggers apoptosis, necrosis and/or permanent cell cycle arrest in the developing early embryo. Embryos deemed to have a low developmental potential of reaching the blastocyst stage in vitro generate significantly elevated levels of intracellular ROS compared to embryos that have a greater tendency for blastocyst formation. Culture regimes utilizing reduced oxygen concentrations or antioxidant supplementation have improved blastocyst development by reducing intracellular ROS production. We have begun to examine the biochemical and molecular pathway that characterize and control cellular redox state in early embryos. Firstly, the early embryo response to ROS is developmentally regulated. Endogenously generated ROS and/or exogenous ROS treatment elicits a dose-dependent, detrimental effect on early development triggering either permanent embryo arrest or apoptosis depending on the preimplantation stage exposed to the oxidative stress. Conversely, antioxidant enzyme supplementation of the culture medium can limitedly reduce ROS-induced developmental failures suggesting that an optimum redox state is required for proper embryo development. We have identified the stress PF-4217903 c-Met inhibitor adaptor protein p66Shc as a major regulator of cellular redox state in early embryos. DNA damage and activated p66Shc. Levels of p66Shc are increased by high atmospheric oxygen or exogenous ROS treatments and are significantly diminished by low oxygen tension or catalase supplementation of the embryo culture media. RNA interference-mediated knockdown of p66Shc in immature bovine oocytes before in vitro fertilization significantly decreased the incidence of permanent embryo arrest, however embryo development to the blastocyst stage was reduced as well. These results suggest that p66Shc mediates early cleavage arrest but is also important for other later events during the preimplantation period. P66Shc is a 66-kDa Src collagen homologue adaptor protein that is one of three main isoforms encoded by the SHC1 gene. While p46Shc and p52Shc isoforms link activated receptor tyrosine kinases to the Ras pathway by recruitment of the GRB2/SOS complex, p66Shc inhibits Ras signaling in response to epidermal growth factor. P66Shc also mediates an inhibitory signaling effect on the extracellular signal-regulated kinase pathway that is required for actin cytoskeleton polymerization and normal glucose transport control. The p66Shc isoform is also involved in signal pathways that regulate the cellular response to oxidative stress and life span. Through phosphorylation of Serine 36 in its unique N-terminal collagen homology-2 domain, p66Shc acts as a downstream target of the tumor suppressor p53 and is essential for the ability of stress-activated p53 to trigger intracellular ROS generation, cytochrome c release, forkhead inactivation.