Acid–base regulation during embryonic development in amniotes, with particular reference to birds

During avian embryonic development, the carbon dioxide tension inside the egg increases as the shell restricts gas exchange with the environment. Acid–base regulation of the avian embryo is a complex process, not only due to the non-function of the lungs and limited functionality of the kidneys but also because the embryo is affected by the inflow of bicarbonates from the shell, when calcium is reabsorbed for calcification. Moreover, interaction occurs between the embryo and the extraembryonic compartments. It has been shown that carbonic anhydrase plays a crucial role in the formation of sub-embryonic fluid and in the chorioallantoic membrane and kidney. This review provides a detailed overview of the acid–base status of the extraembryonic compartments inside chicken eggs, and their interaction in keeping the acid–base status of the embryo balanced. Secondly, experimental acid–base disturbances are summarized. The last part of the review briefly compares embryos of birds and other amniotes (mammals and reptiles) with regard to acid–base regulation.