The role of mesodermal signals during liver organogenesis in zebrafish

Three germ cell layers, the ectoderm, mesoderm and endoderm, are established during the gastrulation stage. All cell types in different organs and tissues are derived from these 3 germ cell layers at later stages. For example, skin epithelial cells and neuronal cells are derived from the ectoderm, while endothelial cells and muscle cells from the mesoderm and lung, and intestine epithelial cells from the endoderm. While in a normal situation different germ cells are destined to specific cell fates in different organs and tissues, each type of germ cells or its derivatives also produce extracellular signaling molecules to direct and facilitate the specification and differentiation of other germ cells during organogenesis. Liver is derived from the endoderm, but completion of liver organogenesis is regulated at different levels. While the pan-endoderm factors (e.g. FoxA and Gata families) and liver specific factors (e.g. Prox1 and Hhex) are essential intrinsic factors for endoderm cells to be differentiated into hepatoblasts, the role of signals produced by neighboring mesoderm cells for liver organogenesis is equally important. This review summarizes recent progress in studying the role of Bone morphogenetic proteins (Bmp), Fibroblast growth factors (Fgf), retinoic acid (RA) and Wingless and Int (Wnt), the 4 types of signaling molecules produced by the mesoderm cells, in liver organogenesis in zebrafish.