Mesendoderm An Ancient Germ Layer?

including the posterior portion of the pharynx, an organ University of Nottingham that has homologies to the heart, coelomocytes, which Queen's Medical Centre resemble macrophages, and a subset of body wall mus-Nottingham NG7 2UH cle. The two GATA factors therefore occupy a pivotal United Kingdom position as zygotic factors specifying a single precursor that gives rise to both mesoderm and endoderm. In much closer relatives of the vertebrates, the sea ur-Formation of the three primary germ layers, ectoderm, chins (echinodermata, a sister phylum to the chordates), mesoderm, and endoderm, is an early distinction be-the macromeres of the 16-cell embryo, and their daugh-tween groups of cells in developing embryos. Our under-ters in the 32-cell embryo, are also mesendodermal, in standing of their generation in vertebrates has benefit-that they give rise to both the endoderm and the majority ted from the classical experiments of Nieuwkoop and of the mesoderm (referenced in Angerer and Angerer, his colleagues (referenced in Nieuwkoop, 1997), in which 2000). What, then, is the evidence that vertebrates de-explants of tissue from the animal hemisphere of am-velop along similar lines? phibian embryos (fated to form ectoderm) apposed to Vertebrate Mesendoderm explants of vegetal tissue (fated to form endoderm) were Labeling of single cells at the mid-blastula stage in ze-induced to form mesoderm. These results have been brafish has shown that endoderm and mesoderm, in-widely interpreted as indicating that mesoderm forms cluding blood, heart, and muscle, derive from cells in the at the interface between presumptive endoderm and 3–4 tiers nearest to the vegetal margin of the blastoderm presumptive ectoderm as a consequence of inductive (Warga and Nusslein-Volhard, 1999 and references therein). signals from the former to the latter. However, recent We will refer to these cells as mesendoderm (Figure 2A). data from nematodes and zebrafish suggest that endo-The mesendodermal nature of these cells is also sug-derm and some portion of the mesoderm may derive gested, in the late blastula, by the coexpression of genes from a bipotential layer of cells, called the mesendo-expressed by and active in the formation of mesoderm derm. In addition, the genes involved in this process and endoderm: the transcription factors, brachyury (bra, may be conserved. also known as no tail (ntl) in the zebrafish) and gata5 In C. elegans, a nematode worm, the endoderm and (faust in the zebrafish) (Rodaway et al., 1999). Perturba-much of the mesoderm derive from the mesendodermal tion of signaling at …