Cardio-respiratory development in bird embryos: new insights from a venerable animal model

The avian embryo is a time-honored animal model for understanding vertebrate development. The embryo is accessible within the shell, independent of direct maternal influence, inexpensive, and easily reared. Research on the avian embryo has elucidated numerous developmental processes representing normal ontogeny, as well as "development gone bad" due to abiotic or biotic perturbations. A key area of extensive study using bird embryos is developmental phenotypic plasticity of the cardio-respiratory system and how its normal development can be affected by abiotic factors such as temperature and oxygen availability. Through the investigation of the plasticity of development, we gain a better understanding of both the regulation of the developmental process and the embryo's capacity for self-repair. Additionally, experiments with abiotic and biotic stressors during development have helped delineate not just critical windows for avian cardio-respiratory development, but the general characteristics (e.g. timing and dose-dependence) of critical windows in all developing vertebrates. Avian embryos are proving useful in exploration of fetal programming, in which early developmental experiences have implications (usually negative) for the organism later in life. The ability to experimentally manipulate the avian embryo without the interference of maternal behavior or physiology makes it particularly useful in future studies of fetal programming. The bird embryo is also a key participant in studies of transgenerational epigenetics, whether by egg provisioning or effects on the germline that are transmitted to the F₁ generation (or beyond). Finally, the avian embryo is heavily exploited in the area of toxicology, where both toxicological testing of potential consumer products as well as the consequences of exposure to anthropogenic pollutants are routinely carried out in the avian embryo. The avian embryo thus proves useful on numerous experimental fronts as an animal model that is concurrently both of adequate complexity and sufficient simplicity for probing vertebrate cardio-respiratory development.