Directed Differentiation of Mesendoderm Derivatives from Embryonic Stem Cells

In amniotes, mesoderm and endoderm arise during gastrulation, the process that derives the three primary germ layers and establishes the basic body plan of the embryo. However, in recent years there has been a new appreciation for a very early stage of development, when some blastomeres are bipotential and may still contribute to either mesoderm or endoderm (but not ectoderm). This tissue has been termed “mesendoderm” (or sometimes “endomesoderm”, but we will use the more common term). Specifically, experiments in nematodes, sea urchins, frogs, or zebrafish showed that when certain single cells were marked at the mid-blastula stage, the labeled cell can contribute to both mesoderm (e.g. blood, heart, muscle) and endoderm (e.g. gut, liver, pancreas) derivatives. Remarkably, the signaling molecules and genetic programs appear to be well conserved across these species (reviewed in Rodaway and Patient, 2001; Wardle and Smith, 2006). Most prominently this involves the nodal signaling pathway (Schier, 2003) and several families of regulatory proteins, including those encoding T-box and GATA transcription factors (Fig. 1). Since zebrafish and frogs are vertebrates, it seems likely that the same developmental programs should function in other vertebrates, including mouse and man. In the mouse, the three germ layers are derived from the epiblast through gastrulation beginning at approximately day 6.5 of gestation. After implantation, the blastocyst, comprising the inner cell mass inside the trophectoderm, develops into an elongated structure composed of the ectoplacental cone, the extraembryonic ectoderm, the visceral endoderm and the epiblast. Gastrulation begins with the formation of a transient structure known as the primitive streak (PS) in the presumptive posterior end of the embryo through which uncommitted epiblast cells mobilize and egress to form the mesoderm and the endoderm (Tam et al., 2007). On the basis of developmental potential and gene expression patterns, the PS can be divided into anterior, mid and posterior regions, with mesoderm developing from the posterior region and the endoderm developing from the most anterior domain. While the close developmental association between endoderm and mesoderm supports the notion that mesendoderm also generates these two germ layers in mammals, the concept is most strongly supported by studies in the embryonic stem cell system (Tada et al., 2005). Mouse embryonic stem (ES) cells generated from the blastocyst inner cell mass can be maintained and expanded as a pure undifferentiated population of cells when grown on mouse feeder cells in media containing leukemia inhibitory factor (LIF) and serum (Evans