Immunohistological analysis on distribution of smooth muscle tissues in livers of various vertebrates with attention to different liver architectures

Abstract

BackgroundThe liver architecture of vertebrates can be classified into two types, the portal triad type (having periportal bile ducts) and the non-portal triad type (having non-periportal bile ducts). The former is detectable from the hagfish, which is the most ancestral vertebrate, to tetrapod livers whereas many actinopterygian livers have the latter. The aim of the present study is to reveal the distribution of smooth muscle tissue in livers of various vertebrates with attention to their architectures. MethodsSmooth muscle was immunohistochemically compared in hepatic blood vessels and bile ducts of various vertebrates, using an anti-alpha-smooth muscle actin (ASMA) antibody. ResultsSmooth muscle was noted in the gallbladder and hepatic artery in all vertebrates, including the hagfish. Bile ducts having ASMA-positive smooth muscles were absent in the hagfish, but detected in the Chondrichthyes and conserved in actinopterygians with or without portal triads during the evolution of vertebrates. In tetrapods having portal triads, reptiles had a tendency to have strongly ASMA-positive biliary smooth muscle tissues whereas other tetrapods had bile ducts with poor smooth muscle tissues. Although the hagfish livers never had ASMA-positive smooth muscle tissue in the walls of portal and central veins, it was observed in discontinuous distributions or not observed in portal veins and central veins of chondrichthyans and actinopterygians. By contrast, in most tetrapods, ASMA-positive smooth muscle tissue was detectable in portal veins, which supported the adjacent endothelial cells as a circular layer. Central veins did not consistently have smooth muscle tissue in these groups. Discussion and conclusionThe hagfish liver may retain more ancestral characteristics than other vertebrates in terms of smooth muscle distribution in the vascular and biliary systems. Actinopterygians might have a different mechanism of bile transport from tetrapods from their smooth muscle distribution in intrahepatic bile ducts. The circular smooth muscle distribution in portal veins might be a characteristic acquired by tetrapods.