Vertebrate head induction by anterior primitive endoderm

In vertebrates the antero-posterior organization of the embryonic body axis is thought to result from the activity of two separate centers, the head organizer and the trunk organizer, as operationally defined by Spemann in the 1920s. Current molecular studies have supported the existence of a trunk organizer activity while the presence of a distinct head inducing center has remained elusive. Mainly based on analyses of headless mutants in mice, it has been proposed that the anterior axial mesoderm plays a determining role in head induction. Recent gain- and loss-of-function studies in various organisms, however, provide compelling evidence that a largely ignored region, the anterior primitive endoderm, specifies rostral identity. In this review we discuss the emerging concept that the anterior primitive endoderm, rather than the prechordal plate mesoderm, induces head development in the vertebrate embryo.     

Vertebrate eye development.

Vertebrate eye determination is mediated by a series of inductive interactions that have now been more precisely defined with the use of regional markers. Analyses of the genes responsible for eye mutations and the cloning of genes delimiting spatial domains within the developing eye have begun to elucidate the molecular basis of this process.     

Primitive endoderm differentiation: from specification to epithelium formation

In amniotes, primitive endoderm (PrE) plays important roles not only for nutrient support but also as an inductive tissue required for embryo patterning. PrE is an epithelial monolayer that is visible shortly before embryo implantation and is one of the first three cell lineages produced by the embryo. We review here the molecular mechanisms that have been uncovered during the past 10 years on PrE and epiblast cell lineage specification within the inner cell mass of the blastocyst and on their subsequent steps of differentiation.     

Vertebrate myotome development

The embryonic myotome generates both the axial musculature and the appendicular muscle of the fins and limbs. Early in embryo development the mesoderm is segmented into somites, and within these the primary myotome forms by a complex series of cellular movements and migrations. A new model of primary myotome formation in amniotes has emerged recently. The myotome also includes the muscle progenitor cells that are known to contribute to the secondary formation of the myotome. The adult myotome contains satellite cells that play an important role in adult muscle regeneration. Recent studies have shed light on how the growth and patterning of the myotome occurs.     

Vertebrate development: Et in Arkadia

The similarities in organiser formation in Xenopus and mouse embryos have remained elusive. Recent evidence suggests a common mechanism, in which an intracellular protein, Arkadia, is required for formation of the mouse organiser and potentiates the effects of the signalling protein Nodal.     

Vertebrate development: the subtle art of germ-layer specification.

Nodal signalling is essential for vertebrate germ-layer formation. How this single signal can generate such a diverse array of tissues remains a mystery and is an area of intense research. Three recent reports reveal unanticipated subtleties to the process and provide new mechanisms for generating distinct responses.     

Vertebrate craniofacial development: novel approaches and new dilemmas.

Unexpected patterns of neuroblast, angioblast and myoblast movement and tissue organization have been defined using lineage tracing and transplantation methods. The most novel and enigmatic new data derive from analyses of genes in the Hox- and Pax-gene families. In addition to the characterization of expression patterns, the effects of Hox-gene knock-out and retinoic acid treatment have been assessed. These basic studies are complemented by the identification of correlations between inherited craniofacial anomalies, for example Waardenburg's syndrome, and the function of specific genes.     

Vertebrate development: taming the nodal waves

Recent results have provided evidence that regulatory loops operating in vertebrate embryos between the developmental signalling factors Nodal and Lefty may provide a real example of the kind of reaction-diffusion process long predicted to be a mechanism of pattern formation.     

Vertebrate development: wnt signals at the crest

Neural crest cells, a defining feature of vertebrate embryos, form at the neural plate border in response to inductive signals from the ectoderm. Recent studies have shown that Wnt signals are essential mediators of this induction.