The Toll/IL-1 receptor binding protein MyD88 is required for Xenopus axis formation

The Toll/Dorsal pathway regulates dorsoventral axis formation in the Drosophila embryo. We had previously obtained evidence that a homologous pathway exists in Xenopus, however, its role during normal frog development had not been established. Here we report the cloning of Xenopus MyD88 (XMyD88), whose mammalian homologs are adaptor proteins linking Toll/IL-1 receptors and IRAK kinases. We show that in the frog embryo overexpression of a dominant-negative form of XMyD88 blocked Toll receptor activity, specifically inhibited axis formation and reduced expression of pivotal organizer genes. The observed stage-dependency of interference suggests a function for maternal XMyD88 soon after fertilization. We conclude that XMyD88 activity is required for normal Spemann organizer formation, implying an essential role for maternal Toll/IL-1 receptors in Xenopus axis formation.     

Yves Delage (1854-1920) as a forerunner of modern nuclear transfer experiments

It is generally considered that animal cloning by nuclear transfer originated in proposals made by Hans Spemann (1936), following his experiments on delayed nucleation in the newt egg, which were preceded by similar attempts using the sea-urchin egg (Loeb, 1894). Briggs and King (1952) were the first to succeed in transplanting blastula and gastrula nuclei into the enucleated frog egg and in obtaining a significant number of normal tadpoles by means of this technique. We present evidence that much earlier (1895) Yves Delage (1854-1920), a French biologist, had clearly formulated the same experimental project of nuclear transfer, as a means to test Weismann's theory of cell differentiation during embryonic development. This was also Spemann's motivation. Both Delage and Spemann were aware of Loeb's experiments (1894), in which delayed nucleation in the sea-urchin egg was found to result in twin larvae. It is difficult to decide whether Delage's project was influenced by Loeb's findings. On the other hand, it seems that Spemann was not aware of Delage's proposal, since he did not express his own ideas on extended nuclear transfer before 1936. Finally, neither Delage nor Spemann imagined that nuclear transfer could be a means of obtaining groups of genetically identical animals (reproductive cloning).     

Bmp signaling: turning a half into a whole

In a famous experiment over a century ago, Hans Spemann demonstrated that amphibians have a remarkable ability to compensate for perturbations to the embryo. In this issue of Cell, Reversade and De Robertis (2005) uncover the basis of this phenomenon. They demonstrate that interactions between bone morphogenetic proteins (Bmps) and their inhibitors on both the dorsal and ventral sides of the early Xenopus embryo are involved in creating the body plan.     

Embryonic induction: Is the Nieuwkoop centre a useful concept?

Regionalisation of the amphibian embryo is classically thought to involve induction by the Spemann organiser, itself induced by the Nieuwkoop centre. This model has now been extended to teleosts, with the identification of a gene that appears to define the zebrafish equivalent of the Nieuwkoop centre.     

Induction and the Turing-Child field in development

The central problem in biological development is the understanding of epigenesis. The dominant theory of development in the last 80 years that also purports to explain epigenesis is induction theory. It suggests that development is driven by sequential inductions where each "induction" (in one sense of the word induction) is effected by the action of an inducing part of the embryo on a responding part of the embryo. The theory stems from Spemann and Mangold (W.Roux' Arch.f.Entw.d.Organis.u.mikrosk.Anat.100 (1924) 599) who transplanted a tissue from the dorsal blastopore lip of Triturus into the ventral ectoderm of another gastrula and thus initiated and "induced" (in another sense of the word induction) gastrulation and embryogenesis in the ventral side of the host that became a double embryo (siamese twins). We explain this induction, i.e. the formation of the double embryo, according to the Child theory and the Turing-Gierer-Meinhardt theory when it is also assumed that cAMP and ATP are the Turing activator and inhibitor, respectively. Spemann and Mangold (W.Roux' Arch.f.Entw.d.Organis.u.mikrosk.Anat.100 (1924) 599) also suggested that the ingressing mesoderm induces the overlying ectoderm to form the neural plate and neural tube. This 'neural induction', the 'primary embryonic induction', became the cornerstone of induction theory, i.e. of the sequential induction concept referred to above. But we argue that the metabolic gradients that precede and accompany neurulation, as obtained by Child, also for Triturus, arise through a Turing self-organization if it is assumed that cAMP and ATP are the Turing morphogens, and these gradients are the cause and primary event of neurulation. Thus there is no need to invoke the 'neural induction'. It is argued that fundamental events such as gastrulation and also organ formation are caused by the Turing-Child field and not by sequential induction. Similar principles, such as bud formation caused by a radial metabolic pattern that transforms to a longitudinal pattern, govern the formation, for example, of the mouth and the gut. The formation and localization of bottle cells is explained according to the Child-Turing field and modern biochemistry. The chemical metabolic pre-pattern precedes, and causes, morphogenesis and differentiation as envisaged by Turing. The Spemann and Mangold (W.Roux' Arch.f.Entw.d.Organis.u.mikrosk.Anat.100 (1924) 599) transplantation experiment when performed on a sea urchin duplicates not only the phenotype but also the metabolic (reduction) pattern. These experimental results, by Horstadius, predicted by Child, follow from the Turing-Gierer-Meinhardt theory if it is assumed that cAMP and ATP are the Turing morphogens. If the transplantation is performed not onto the whole sea urchin but onto only a part of it, that manifests only a part of the metabolic pattern, then from the part a phenotypic whole underlain by a normal and a whole metabolic pattern can be rescued. These experimental results of Horstadius follow from Turing theory if cAMP and ATP are the Turing morphogens. Understanding how to transform a part into a whole can be valuable in regenerative medicine. Unspecific induction of a secondary amphibian embryo is similar to the induction of posterior structures at the anterior pole of an insect, and the "double abdomen" (and Kalthoff's experimental results) of the midge Smittia resulting from UV irradiation of the anterior pole, can be explained by Meinhardt theory of unspecific induction if ATP is the Turing morphogen. When not working on regeneration, Child investigated intact living organisms and his observation method was not disruptive to normal development, whereas workers in induction theory work with pieces and in general disrupt normal development. We conclude that the Turing-Child field causes all development and explains epigenesis. Sequential induction does not explain epigenesis and does not exist in normal development. But induction in the sense of a transplantation leading to double embryo or rescuing a whole phenotype from a part is of interest.     

Spemann's organizer and self-regulation in amphibian embryos

In 1924, Spemann and Mangold demonstrated the induction of Siamese twins in transplantation experiments with salamander eggs. Recent work in amphibian embryos has followed their lead and uncovered that cells in signalling centres that are located at the dorsal and ventral poles of the gastrula embryo communicate with each other through a network of secreted growth-factor antagonists, a protease that degrades them, a protease inhibitor and bone-morphogenic-protein signals.     

Regulation of early expression of Dlx3, a Xenopus anti-neural factor, by beta-catenin signaling

The ectoderm of the pre-gastrula Xenopus embryo has previously been shown to be at least partially patterned along the dorsal-ventral axis. The early expression of the anti-neural homeodomain gene Dlx3 is localized to the ventral ectoderm by a mechanism that occurs prior to gastrulation and is independent of the Spemann organizer. The repression of Dlx3 is mediated by signaling though beta-catenin, but is probably not dependent on the induction of the Xnr3 or chordin genes by beta-catenin. We propose a model in which this early regulation of Dlx3 accounts for the pro-neural bias of dorsal ectoderm.     

Introducing the Spemann-Mangold organizer: experiments and insights that generated a key concept in developmental biology

The "organizer paper", published by Hans Spemann and Hilde Mangold in 1924, initiated a new epoch in developmental biology. Also it marked the climax of Spemann's life-long research which began at the end of the nineteenth century. This introduction retraces some of the steps by which Spemann arrived at the organizer concept: The problem of amphibian lens induction including the so-called lens controversy, the early constriction experiments creating double headed malformations, and the homeo- and heteroplastic transplantations during gastrula stages of the newt. Furthermore this paper will--based on historical documents--repudiate some objections raised to the contribution of Spemann and Hilde Mangold to the discovery and interpretation of the organizer effect.     

The Spemann-Mangold organizer: the control of fate specification and morphogenetic rearrangements during gastrulation in Xenopus

Vertebrate embryonic development is controlled by sequentially operating signalling centres that organize spatial pattern by inductive interactions. The embryonic body plan is established during gastrulation through the action of the Spemann-Mangold or gastrula organizer, a signalling source discovered 75 years ago by Hans Spemann and Hilde Mangold. Transplantation of the organizer to a heterotopic location in a recipient embryo results in the formation of a secondary embryonic body axis, in which several tissue types, most notably somites and the neural tube, are derived from ventral host cells. Because of these non-cell autonomous recruiting or inducing activities the organizer has become a paradigm for studying intercellular communication in the vertebrate embryo. Here, I review some of the recent advances in understanding 1) the initiation of the Spemann-Mangold organizer, 2) its function in pattern formation along the dorsal-ventral and anterior-posterior axes and 3) the integration of cell fate specification events and downstream execution of morphogenetic movements during gastrulation in Xenopus laevis.     

A Pact with the Embryo: Viktor Hamburger,Holistic and Mechanistic Philosophy in the Development of Neuroembryology, 1927–1955

Viktor Hamburger was a developmental biologist interested in the ontogenesis of the vertebrate nervous system. A student of Hans Spemann at Freiburg in the 1920s, Hamburger picked up a holistic view of the embryo that precluded him from treating it in a reductionist way; at the same time, he was committed to a materialist and analytical approach that eschewed any form of vitalism or metaphysics. This paper explores how Hamburger walked this thin line between mechanistic reductionism and metaphysical vitalism in light of his work on the factors influencing growth of neurons into limb buds, and the discovery of nerve growth factor, work carried out with Rita Levi-Montalcini and Stanley Cohen.     

Amphibian embryos as a model system for organ engineering: in vitro induction and rescue of the heart anlage.

Beating hearts can be induced under in vitro conditions when the dorsal blastopore lip (including the zone of Spemann organizer) is treated with Suramin. In contrast, untreated organizer forms dorsal mesodermal derivatives as notochord and somites. When those in vitro produced heart precursor tissues are transplanted ectopically in the posterior trunk area of early larvae, secondary beating heart structures will be formed. Furthermore, the replacement of the heart primordium of the host embryo by heart tissue induced under in vitro conditions will result in the rescue of the heart anlage. This model could be a valuable tool for the study of the multi-step molecular mechanisms of heart structure induction under in vitro conditions and vasculogenesis after transplantation into the host embryo.     

The impacts of overwintered green frog larvae on wood frog embryo mortality under a wetter climate

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Facial Transplants in Xenopus laevis Embryos

Craniofacial birth defects occur in 1 out of every 700 live births, but etiology is rarely known due to limited understanding of craniofacial development. To identify where signaling pathways and tissues act during patterning of the developing face, a ''face transplant'' technique has been developed in embryos of the frog Xenopus laevis. A region of presumptive facial tissue (the "Extreme Anterior Domain" (EAD)) is removed from a donor embryo at tailbud stage, and transplanted to a host embryo of the same stage, from which the equivalent region has been removed. This can be used to generate a chimeric face where the host or donor tissue has a loss or gain of function in a gene, and/or includes a lineage label. After healing, the outcome of development is monitored, and indicates roles of the signaling pathway within the donor or surrounding host tissues. Xenopus is a valuable model for face development, as the facial region is large and readily accessible for micromanipulation. Many embryos can be assayed, over a short time period since development occurs rapidly. Findings in the frog are relevant to human development, since craniofacial processes appear conserved between Xenopus and mammals.     

Developmental biology of amphibians after Hans Spemann in Germany

After the Hans Spemann and Hilde Mangold discovery of the importance of the dorsal blastopore lip for axis formation in the early embryo (Nobelprize for Spemann, 1935), the scientific community tried in a goldrush-like manner to find the inducing factors responsible for the programming of early embyronic determination and differentiation. The slow progress towards a solution of this problem caused a fading of interest on behalf of most laboratories. This article describes the activities of a few laboratories in Finland, Japan and Germany, which continued their studies despite tremendous experimental difficulties. Finally only Heinz Tiedemann's group in Berlin was the first which could isolate a mesoderm/endoderm inducing factor in highly purified form, the so-called vegetalizing factor, now known as activin. Furthermore this article describes the identification of neuralizing factors like Chordin, Cerberus and Dickkopf in the zone of the Spemann-Mangold organizer. The finding that BMP-4 acts as an antagonist to these factors located on the dorsal side led to a new understanding of the mechanisms of action of inducing (neuralizing) factors and early embryonic pattern formation. Moreover, the observations that closely related genes and their products were also found in Drosophila, Zebrafish, Mice and Human were the basis for new concepts of evolutionary mechanisms (dorsal/ventral and anterior/posterior polarity or conserved processes in eye-development of all 7 animal phyla).     

Induction into the Hall of Fame: tracing the lineage of Spemann's organizer

The grafting experiments of Spemann and Mangold have been a textbook classic for years, but as with many conclusions from experimental embryology, the idea that the dorsal lip of the blastopore ;organized' the early patterning of the embryo has sometimes come under question. In their 1983 paper in JEEM, Smith and Slack extended these classical experiments in newts to the now-standard amphibian model Xenopus laevis. By using injected lineage tracers, they distinguished the fates of graft and host, and showed unambiguously that the organizer is responsible for neural induction and that it dorsalizes the mesoderm.     

Photoreactivation of a Cytoplasmic Virus

Ultraviolet light-inactivated frog virus 3 is efficiently photoreactivated by chick embryo cells. A cellular enzyme is presumably responsible for this repair of viral deoxyribonucleic acid, for the phenomenon is insensitive to an inhibitor of protein synthesis and is not seen in mammalian cells that are known to lack photoreactivating enzyme. Since frog virus 3 is a cytoplasmic virus, functionally significant amounts of photoreactivating enzyme are probably present in the cytoplasm of chick embryo cells.