Hepatic caecum of amphioxus and origin of vertebrate liver

Liver is characteristic of all vertebrates.As a critical hub for many physiological processes including metabolism,innate immunity,protein synthesis and detoxification,its evolutionary origin was largely underappreciated in history,and only received due attention in recent decades.It has been suggested by morphological,ultrastructural and immunohistochemical studies that the hepatic caecum of amphioxus is homologous to the liver of vertebrate species.Molecular biology studies demonstrated that amphioxus hepatic caecum expresses plenty of vertebrate liver-specific genes.Our functional studies revealed significant similarities between amphioxus hepatic caecum and vertebrate liver.We also found that the functions of hepatic caecum are subjected to the regulation of pituitary hormones just as the liver does.These provide solid evidences supporting the notion that the hepatic caecum is the homologue of liver,which may represent the first stage in chordate evolution,laying a foundation for the subsequent formation of the liver as we know it in vertebrates.Further studies on the specification and morphogenesis of hepatic caecum in amphioxus will shed more lights on the origin and evolution of vertebrate liver.