Morphogenesis of chicken liver: identification of localized growth zones and the role of beta-catenin/Wnt in size regulation.

During development and regeneration, new cells are added and incorporated to the liver parenchyma. Regulation of this process contributes to the final size and shape of the particular organs, including the liver. We identified the distribution of liver growth zones using an embryonic chicken model because of its accessibility to experimentation. Hepatocyte precursors were first generated all over the primordia surrounding the vitelline blood vessel at embryonic day 2 (E2), then became limited to the peripheral growth zones around E6. Differentiating daughter cells of the peripheral hepatocyte precursors were shown by DiI microinjection to be laid inward and were subsequently organized to form the hepatic architecture. At E8, hepatocyte precursor cells were further restricted to limited segments of the periphery, called localized growth zones (LoGZ). Adhesion and signaling molecules in the growth zone were studied. Among them, beta-catenin and Wnt 3a were highly enriched. We overexpressed constitutively active beta-catenin using replication competent avian sarcoma (RCAS) virus. Liver size increased about 3-fold with an expanded hepatocyte precursor cell population. In addition, blocking beta-catenin activity by either overexpression of dominant-negative LEF1 or overexpression of a secreted Wnt inhibitor Dickkopf (DKK) resulted in decreased liver size with altered liver shape. Our data suggest that (1) the duration of active growth zone activity modulates the size of the liver; (2) a shift in the position of the localized growth zone helps to shape the liver; and (3) beta-catenin/Wnt are involved in regulating growth zone activities during liver development.