Even with the endogenous defense mechanisms present in oocytes and embryos, disturbances in embryonic culture processes may lead to an increased generation and buildup of reactive oxygen species (ROS) in culture media. This fact may be associated with various degrees of cellular damage (e.g. membrane disturbances, DNA fragmentation, etc.). Consequently, embryonic fragmentation, apoptosis, or even slowness or interruption of embryonic development is frequently seen. ROS may stem directly from gametes and embryos or in the environment in which they are found. The exposure of oocytes and embryos to xenobiotic agents, changes in metabolic substrate concentrations, and traces of transition elements favor the generation of ROS during in vitro culture/manipulation. High concentrations of oxygen is also listed as a harmful factor in this process.