This article is about how the famous organizer experiment has been perceived since it was first published in 1924. The experiment involves the production of a secondary embryo under the influence of a graft of a dorsal lip from an amphibian gastrula to a host embryo. The early experiments of Spemann and his school gave rise to a view that the whole early amphibian embryo was \"indifferent\" in terms of determination, except for a special region called \"the organizer\". This was viewed mainly as an agent of neural induction, also having the ability to generate an anteroposterior body pattern. Early biochemical efforts to isolate a factor emitted by the organizer were not successful but culminated in the definition of \"neuralizing (N)\" and \"mesodermalizing (M)\" factors present in a wide variety of animal tissues. By the 1950s this view became crystallized as a \"two gradient\" model involving the N and M factors, which explained the anteroposterior patterning effect. In the 1970s, the phenomenon of mesoderm induction was characterized as a process occurring before the commencement of gastrulation. Reinvestigation of the organizer effect using lineage labels gave rise to a more precise definition of the sequence of events. Since the 1980s, modern research using the tools of molecular biology, combined with microsurgery, has explained most of the processes involved. The organizer graft should now be seen as an experiment which involves multiple interactions: dorsoventral polarization following fertilization, mesoderm induction, the dorsalizing signal responsible for neuralization and dorsoventral patterning of the mesoderm, and additional factors responsible for anteroposterior patterning.

Understanding embryogenesis currently relies largely on the control of gene expression via several signaling pathways. Many of the embryonic signaling pathways guiding embryological events are implicated in diseases that lack effective cure or treatment. Because of the large number and size of the eggs, the rapid development of the embryos and the fact they are amenable to pharmacological, surgical and genetic techniques, Xenopus laevis has been successfully used in searching for compounds that target embryonic signaling pathways. Areas covered: Here, the authors address the use of amphibian eggs/embryos in successful chemical screenings; egg extracts as well as embryo phenotypes have been assayed to reveal drug toxicology effects and novel compounds acting in the Wnt/β-catenin signaling pathway. They do not discuss the use of Xenopus oocyte two-electrode voltage clamps or genome editing tools as approaches for drug discovery because they have been discussed elsewhere. Expert opinion: While high-throughput screening is commonly performed in egg extracts, the embryo axes perturbation system is more suited to the refinement and/or the validation of drug discovery targeting embryonic signaling (particularly the Wnt/β-catenin pathway). In addition, Xenopus has also been used in FETAX (frog embryo teratogenesis assay: Xenopus) to address chemical toxic/teratogenic effects. However, further studies are necessary.

The Serum Response Element (SRE) is a sequence motif which activates transcription of certain genes in response to factors that stimulate cell proliferation. This motif is found in the promoter of aXenopus laevis cytoskeletal actin gene, which is transcriptionally activated very early in embryonic development. We tested whether the SRE plays a role in the developmentally-timed transcriptional activation of this gene by constructing an SRE replacement mutant and studying its transcription after microinjection intoXenopus embryos. Normal amounts of actin mRNA are transcribed at the normal time in development from this mutant, suggesting that the SRE is not the sole determinant of temporal specificity of actin gene transcription in the embryo.

The neural-inducing activity of artificially mesodermalized ectoderm was examined. The competent ectoderm of earlyCynops gastrula was mesodermalized by being placed in contact withCarassius swimbladder. The mesodermalized ectoderm was combined with ectoderm isolated from various developmental stages of a gastrula. Neural differentiation were observed in half the combinants, even in 18 h ectoderm, which is considered to have lost its neural competence within 6 h. This indicates that mesodermalized ectoderm is capable of inducing neural tissues at the very time it comes into contact with 18 h ectoderm. From the present study, the neural-inducing activity of mesodermalized cells may possibly be closely connected to the early process of their mesodermalization.

Explants comprising about 15 cells were dissected from various regions of the blastula ofAmbystoma mexicanum and cultured in Barth\'s medium. By addition of L-tyrosine to the culture medium it was possible to induce melanin synthesis in three different cells types: undifferentiated embryonic cells, mesenchyme cells and nerve cells. Tyrosine was found to act as an inductor in a very low concentration (1 μM). It is suggested that tyrosine serves both as an inductor and as a substrate for melanin synthesis in the amphibian larva.

Embryos of many animal models express germ line determinants that suppress transcription and mediate early germ line commitment, which occurs before the somatic cell lineages are established. However, not all animals segregate their germ line in this manner. The \'last cell standing\' model describes primordial germ cell (PGC) development in axolotls, in which PGCs are maintained by an extracellular signalling niche, and germ line commitment occurs after gastrulation. Here, we propose that this \'stochastic\' mode of PGC specification is conserved in vertebrates, including non-rodent mammals. We postulate that early germ line segregation liberates genetic regulatory networks for somatic development to evolve, and that it therefore emerged repeatedly in the animal kingdom in response to natural selection.

The LC10, 50 and 90/24h of aluminum for Rhinella arenarum embryos at complete operculum stage were 0.55, 0.75 and 1mgAl(3+)/L respectively. Those values did not change significantly by expanding the exposure period till 168h. The aluminum toxicity was evaluated in different pH conditions by means of a citrate buffer resulting for instance, 1mgAl(3+)/L at pH 4, 4.1, 5 and 6 in 100%, 70%, 35% and 0% of lethality respectively. As an outstanding feature, the embryos changed the pH of the maintaining media both in the case of Al(3+) or citrate buffer treatments toward neutral. 10 embryos in 40mL of AMPHITOX solution were able to increase the pH from 4.2 to 7.05, a fact related with a metabolic shift resulting in an increase in nitrogen loss as ammonia. Our study point out the natural selection of the most resistant amphibian embryos both for pH or aluminum as well as the capacity of living organisms (as a population) to alter their chemical environment toward optimal conditions for their survival. As these facts occur at early life stages, it expand the concept that living organisms at ontogenic stages are biomarker of environmental signatures of the evolutionary process (Herkovits, 2006) to a global Onto-Evo concept which imply also the feedback mechanisms from living organisms to shape environmental conditions in a way that benefits them.