The liver plays an indispensable part in many processes in the body, particularly those concerned with its metabolism (e.g., protein synthesis, storage metabolites, bile secretion and detoxification) that shoulder a central role into maintaining life, and with certain digestive processes. It is the organ in which nutrients absorbed in the digestive tract are processed and stored for use by other parts of the body.
The usual concept of structural and functional unit of the liver is the acinus, containing both the hepatic lobule and portal triad. The hepatic lobule is formed hepatocyte-sinusoidal structures in which consist of both hepatocytes and sinusoids. The sinusoids are capillary networks and are localized in the space between hepatic plates in which hepatocytes are arranged . In mammals, hepatic plates line simple-layered hepatocytes, so-called one-cell-thick plates or with a cord-like form . In teleosts, hepatic plates line the multi-layered hepatocytes, so-called two- or several-cell-thick plates and/or solid or tubular types [2, 3].
The portal triads are located in the portal spaces between the hepatic lobules and contain branches of the portal vein and hepatic artery, bile duct and lymph vessels which are surrounded by connective tissue. In amphibians, the liver of the newt possesses immunologic capabilities due to the presence of lymphocytes in both the connective tissue region in the portal triad and the perihepatic subcapsular region [4, 5]. It is the site of formation of lymphocytes and of the eosinophil leukocytes. In contrast, mice and humans, except the fetal liver, hematopoietic tissue structures are not possessed in these regions. The fetal liver has the initial site of fetal hematopoiesis [6, 7] and B cell development in mammals .
In amphibian livers, a number of morphological studies have been performed. The recent aims of the amphibian liver have been as follows: (1) animal diversity and evolution (e.g., phylogeny, ontogeny, and taxonomy), (2) immunological mechanism (e.g., lymphoid system and pigment system), and (3) pollution (e.g., endocrine disruptors). Evolutionary or phylogenetic relationships among the families of living amphibians are basic to an interpretation of their biography and to constructing a meaningful classification. The current zoological viewpoints have been focused and investigated in the themes of biodiversity or evolution, but there has been little phylogenic research into any vertebrates in liver evolution [9–16]. On the other hand, the interaction of hepatocyte-sinusoidal structures with phylogeny in several vertebrate species has been elucidated [2, 3]; however, there is no study among each order in amphibians.
Amphibians can be grouped into three orders: Gymnophiona, Caudata and Anura [17–19]. Gymnophiona are elongate, legless, wormlike animals that live primarily in tropical areas. Caudata include newts and salamanders, and newts are aquatic members of the Salamandridae family. Anurans include tailless toads and frogs. The adults of most species are terrestrial, although the genus Xenopus is an aquatic member of the Pipidae family .
The origin and divergence of the three living orders of amphibians (Gymnophiona, Caudata, Anura) and their main lineages are one of the most hotly debated topics in vertebrate evolution . A phylogenic study of amphibian livers may be valid as an optimal model for liver ontogenesis in vertebrates. To demonstrate the correlation between liver structures and phylogenic status, we observed 46 amphibian livers by light microscope, and subjected the data to phylogenic analyses. We focused on the architecture of hepatocyte-sinusoidal structures and hematopoietic tissue structures.