Embryonic development of the liver†

Abstract

The combined use of genetic manipulation in mice and advanced culturing techniques, along with the availability of model genetic systems, underlies the recent expansion in our understanding of the fundamental processes that govern the development of the liver. Although gaps in our knowledge still exist, many of the molecules and pathways that act during the earliest stages of hepatic development have been identified. This knowledge has been applied to control the differentiation of embryonic stem and hepatic progenitor cells into hepatocytes and cholangiocytes, raising the prospect of generating quantities of hepatic cells in culture that are suitable for therapeutic uses. mRNA, messenger RNA; E, embryonic day; FGF, fibroblast growth factor; BMP, bone morphogenetic protein. The principal functional cell type of the liver is the hepatocyte, which accounts for 78% of parenchymal volume in the rat liver.1 The hepatocyte is a polarized epithelial cell that exhibits both endocrine and exocrine properties. Biliary epithelial cells (cholangiocytes), sinusoidal endothelial cells, hepatic stellate cells (Ito cells), Kupffer cells, and pit cells (liver-specific natural killer cells) represent the majority of non-hepatocyte cell types in the liver. Although most research has traditionally focused on the hepatocyte, it is now clear that non-hepatocyte cell types make crucial contributions to liver function and pathology, and efforts to understand their embryonic origins and the mechanisms underlying their development have intensified.2 In addition, there is mounting evidence to support the proposal that some nonparenchymal cells have a direct impact on the development of hepatocytes.3-5 For example, hepatoblasts purified from fetal mouse liver have been shown to require nonparenchymal cells for proliferation and expression of hepatocyte messenger RNA (mRNA).4 To understand how the liver is generated during embryogenesis, it is first necessary to define the lineage relationships and the morphogenic movements of the cells that contribute to the liver's function. Several decades of work in numerous vertebrate model systems have firmly established that hepatocytes and bile duct cells originate from a common precursor, the hepatoblast, that derives from the definitive endoderm (Fig. 1).6-13 At around embryonic day (E) 7.0 in the mouse, definitive endoderm emerges from the primitive streak to displace the extra-embryonic endoderm of the yolk sac. Shortly after this, the endoderm invaginates to form a portal at the anterior region of the developing embryo that will ultimately define the foregut of the mouse. By approximately E8.0, in mouse embryos containing seven somite pairs, the ventral wall of the foregut endoderm is positioned adjacent to the developing heart, and signals from the heart induce the underlying endoderm to initiate its development toward a hepatic fate.6, 9, 14, 15 The endoderm responds to this induction by generating the primary liver bud that can be identified as an anatomical outgrowth from the ventral wall of the foregut by E8.5 to E9.0 in mouse embryos containing 10 to 12 somite pairs. Recently, lineage tracing experiments in cultured mouse embryos have accurately defined the position of endodermal cells that give rise to the hepatic progenitors prior to their interaction with the heart.16 Two domains of endodermal cells were identified when E8.5 embryos containing one to three somite pairs were labeled: a lateral endodermal cluster that migrates toward the midline as the foregut is generated, and ventral midline endodermal lip cells that grow caudally to generate the midline of the ventral aspect of the gut. By E9.5, in mouse embryos containing approximately 15 to 20 somite pairs, the basement membrane surrounding the liver bud is lost, and cells delaminate from the bud and invade the surrounding septum transversum mesenchyme as cords of hepatoblasts (see Fig. 1; Fig. 2A-B). Expression of a number of proteins has been used to identify cells undergoing this phase of hepatic development, including expression of the transcription factors HNF4α and GATA4, which identify the migrating hepatic cells and the septum transversum mesenchyme, respectively (see Fig. 2). Although the molecular mechanisms that control the onset of delamination and migration of the hepatoblasts are not well defined, the process does require the transcription factor prospero-related homeobox 1 (Prox1).17 Examination of Prox1−/− embryos revealed that hepatoblasts failed to migrate and invade the septum transversum mesenchyme, and the mutant cells remained clustered and surrounded by basement membrane. The Prox1−/− hepatoblasts also expressed abnormally high levels of the cell adhesion molecule E-cadherin, which may contribute to the inability of these cells to fully delaminate.17 Surprisingly, the Prox1−/− embryos did generate liver lobes that were devoid of hepatocytes.17 This finding implies that the mesenchymal component of the liv