Organizing chordates with an organizer

Understanding how the chordate body plan originated and evolved is still controversial. The discovery by Spemann and Mangold in 1924 of the vertebrate organizer and its inductive properties in patterning the AP and DV axis was followed by a long gap until the 1960s when scientists started characterizing the molecular events responsible for such inductions. However, the evolutionary origin of the organizer itself remained obscure until very recently; did it appear together with the origin and radiation of vertebrates, or was it a chordate affair? A recent study by Yu and collaborators, 1 which analyses the expression of several organizer-specific genes in amphioxus together with recent phylogenetic data that reversed the position of invertebrate extant chordates (e.g. urochordates and cephalochordates), indicates that the organizer probably appeared in early chordates. It likely had separate signalling centres generating BMP and Wnt signalling gradients along the DV and AP axis. The organizer was then lost in the urochordate lineage, most probably as an adaptation to a rapid and determinate development. BioEssays 29:619–624, 2007. © 2007 Wiley Periodicals, Inc.     

Hemichordates and the origin of chordates

Hemichordates, the phylum of bilateral animals most closely related to chordates, could reveal the evolutionary origins of chordate traits such as the nerve cord, notochord, gill slits and tail. The anteroposterior maps of gene expression domains for 38 genes of chordate neural patterning are highly similar for hemichordates and chordates, even though hemichordates have a diffuse nerve-net. About 40% of the domains are not present in protostome maps. We propose that this map, the gill slits and the tail date to the deuterostome ancestor. The map of dorsoventral expression domains, centered on a Bmp-Chordin axis, differs between the two groups; hemichordates resemble protostomes more than they do chordates. The dorsoventral axis might have undergone extensive modification in the chordate line, including centralization of the nervous system, segregation of epidermis, derivation of the notochord, and an inversion of organization.     

The deuterostome ancestor

Hemichordates, the phylum of bilateral animals closest to chordates, can illuminate the evolutionary origins of various chordate traits to determine whether these were already present in a shared ancestor (the deuterostome ancestor) or were evolved within the chordate line. We find that an anteroposterior map of gene expression domains, representing 42 genes of neural patterning, is closely similar in hemichordates and chordates, though it is restricted to the neural ectoderm in chordates whereas in hemichordates, which have a diffuse nervous system, it encircles the whole body. This map allows an accurate alignment of the anterioposterior axes of members of the two groups. We propose that this map dates back at least to the deuterostome ancestor. The map of dorsoventral expression domains, organized along a Bmp-Chordin developmental axis, is also similar in the two groups in terms of many gene expression domains and for the placement of the gill slits, heart, and post-anal tail. The two groups, however, differ in two major respects along this axis. The nervous system and epidermis are not segregated into distinct territories in hemichordates, as they are in chordates, and furthermore, the mouth is on the Chordin side in hemichordates but the Bmp side in chordates. The dorsoventral dimension has undergone extensive modification in the chordate line, including centralization of the nervous system, segregation of epidermis, derivation of the notochord, perhaps from the gut midline, and relocation of the mouth. Based on the shared domain maps, speculations can be made for the remodeling of the body axis in the chordate line.     

Head in the WNT: the molecular nature of Spemann's head organizer.

Formation of the head during vertebrate embryogenesis has been one of the most-studied topics in development, probably because we are such cephalized beings ourselves. Early experimenters found that the head is induced during gastrulation by Spemann's organizer. In 1999 we celebrate the 75th anniversary of the discovery of the organizer by Spemann and Mangold, a group of cells in amphibia that secretes powerful signalling molecules. Recently, advances have been made in identifying candidate head inducers. Not surprisingly, these inducers act in familiar molecular pathways, namely transforming growth factor beta (TGF-beta) and WNT signalling.     

The origins of axial patterning in the metazoa: how old is bilateral symmetry?

Bilateral symmetry is a hallmark of the Bilateria. It is achieved by the intersection of two orthogonal axes of polarity: the anterior-posterior (A-P) axis and the dorsal-ventral (D-V) axis. It is widely thought that bilateral symmetry evolved in the common ancestor of the Bilateria. However, it has long been known that members of the phylum Cnidaria, an outgroup to the Bilateria, also exhibit bilateral symmetry. Recent studies have examined the developmental expression of axial patterning genes in members of the phylum Cnidaria. Hox genes play a conserved role in patterning the A-P axis of bilaterians. Hox genes are expressed in staggered axial domains along the oral-aboral axis of cnidarians, suggesting that Hox patterning of the primary body axis was already present in the cnidarian-bilaterian ancestor. Dpp plays a conserved role patterning the D-V axis of bilaterians. Asymmetric expression of dpp about the directive axis of cnidarians implies that this patterning system is similarly ancient. Taken together, these result imply that bilateral symmetry had already evolved before the Cnidaria diverged from the Bilateria.     

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.     

Gastrulation and pre-gastrulation morphogenesis, inductions, and gene expression: similarities and dissimilarities between urodelean and anuran embryos

Studies of meso-endoderm and neural induction and subsequent body plan formation have been analyzed using mainly amphibians as the experimental model. Xenopus is currently the predominant model, because it best enables molecular analysis of these induction processes. However, much of the embryological information on these inductions (e.g., those of the Spemann-Mangold organizer), and on the morphogenetic movements of inductively interacting tissues, derives from research on non-model amphibians, especially urodeles. Although the final body pattern is strongly conserved in vertebrates, and although many of the same developmental genes are expressed, it has become evident that there are individually diverse modes of morphogenesis and timing of developmental events. Whether or not this diversity represents essential differences in the early induction processes remains unclear. The aim of this review is to compare the gastrulation process, induction processes, and gene expressions between a urodele, mainly Cynops pyrrhogaster, and an anura, Xenopus laevis, thereby to clarify conserved and diversified aspects. Cynops gastrulation differs significantly from that of Xenopus in that specification of the regions of the Xenopus dorsal marginal zone (DMZ) are specified before the onset of gastrulation, as marked by blastopore formation, whereas the equivalent state of specification does not occur in Cynops until the middle of gastrulation. Detailed comparison of the germ layer structure and morphogenetic movements during the pre-gastrula and gastrula stages shows that the entire gastrulation process should be divided into two phases of notochord induction and neural induction. Cynops undergoes these processes sequentially after the onset of gastrulation, whereas Xenopus undergoes notochord induction during a series of pre-gastrulation movements, and its traditionally defined period of gastrulation only includes the neural induction phase. Comparing the structure, fate, function and state of commitment of each domain of the DMZ of Xenopus and Cynops has revealed that the true form of the Spemann-Mangold organizer is suprablastoporal gsc-expressing endoderm that has notochord-inducing activity. Gsc-expressing deep endoderm and/or superficial endoderm in Xenopus is involved in inducing notochord during pre-gastrulation morphogenesis, rather than both gsc- and bra-expressing tissues being induced at the same time.     

Polarized basolateral cell motility underlies invagination and convergent extension of the ascidian notochord.

We use 3D time-lapse analysis of living embryos and laser scanning confocal reconstructions of fixed, staged, whole-mounted embryos to describe three-dimensional patterns of cell motility, cell shape change, cell rearrangement and tissue deformation that accompany formation of the ascidian notochord. We show that notochord formation involves two simultaneous processes occurring within an initially monolayer epithelial plate: The first is invagination of the notochord plate about the axial midline to form a solid cylindrical rod. The second is mediolaterally directed intercalation of cells within the plane of the epithelial plate, and then later about the circumference of the cylindrical rod, that accompanies its extension along the anterior/posterior (AP) axis. We provide evidence that these shape changes and rearrangements are driven by active extension of interior basolateral notochord cell edges directly across the faces of their adjacent notochord neighbors in a manner analogous to leading edge extension of lamellapodia by motile cells in culture. We show further that local edge extension is polarized with respect to both the AP axis of the embryo and the apicobasal axis of the notochord plate. Our observations suggest a novel view of how active basolateral motility could drive both invagination and convergent extension of a monolayer epithelium. They further reveal deep similarities between modes of notochord morphogenesis exhibited by ascidians and other chordate embryos, suggesting that cellular mechanisms of ascidian notochord formation may operate across the chordate phylum.     

Unusual number and genomic organization of Hox genes in the tunicate Ciona intestinalis

Hox genes are organized in genomic clusters. In all organisms where their role has been studied, Hox genes determine developmental fate along the antero-posterior axis. Hence, these genes represent an ideal system for the understanding of relationships between the number and expression of genes and body organization. We report in this paper that the ascidian Ciona intestinalis genome appears to contain a single Hox gene complex which shows absence of some of the members found in all chordates investigated up to now. Furthermore, the complex appears to be either unusually long or split in different subunits. We speculate that such an arrangement of Hox genes does not correspond to the chordate primordial cluster but occurred independently in the ascidian lineage.     

Molecular interactions continuously define the organizer during the cell movements of gastrulation.

The organizer is a unique region in the gastrulating embryo that induces and patterns the body axis. It arises before gastrulation under the influence of the Nieuwkoop center. We show that during gastrulation, cell movements bring cells into and out of the chick organizer, Hensen's node. During these movements, cells acquire and lose organizer properties according to their position. A "node inducing center," which emits Vg1 and Wnt8C, is located in the middle of the primitive streak. Its activity is inhibited by ADMP produced by the node and by BMPs at the periphery. These interactions define the organizer as a position in the embryo, whose cellular makeup is constantly changing, and explain the phenomenon of organizer regeneration.     

Neural induction and patterning in the mouse in the absence of the node and its derivatives

The signals which induce vertebrate neural tissue and pattern it along the anterior-posterior (A-P) axis have been proposed to emanate from Spemann's organizer, which in mammals is a structure termed the node. However, mouse embryos mutant for HNF3 beta lack a morphological node and node derivatives yet undergo neural induction. Gene expression domains occur at their normal A-P axial positions along the mutant neural tubes in an apparently normal temporal manner, including the most anterior and posterior markers. This neural patterning occurs in the absence of expression of known organizer genes, including the neural inducers chordin and noggin. Other potential signaling centers in gastrulating mutant embryos appear to express their normal constellation of putative secreted factors, consistent with the possibility that neural-inducing and -patterning signals emanate from elsewhere or at an earlier time. Nevertheless, we find that the node and the anterior primitive streak, from which the node derives, are direct sources of neural-inducing signals, as judged by expression of the early midbrain marker Engrailed, in explant-recombination experiments. Similar experiments showed the neural-inducing activity in HNF3 beta mutants to be diffusely distributed. Our results indicate that the mammalian organizer is capable of neural induction and patterning of the neural plate, but that maintenance of an organizer-like signaling center is not necessary for either process.     

The Spemann organizer meets the anterior‐most neuroectoderm at the equator of early gastrulae in amphibian species

The dorsal blastopore lip (known as the Spemann organizer) is important for making the body plan in amphibian gastrulation. The organizer is believed to involute inward and migrate animally to make physical contact with the prospective head neuroectoderm at the blastocoel roof of mid‐ to late‐gastrula. However, we found that this physical contact was already established at the equatorial region of very early gastrula in a wide variety of amphibian species. Here we propose a unified model of amphibian gastrulation movement. In the model, the organizer is present at the blastocoel roof of blastulae, moves vegetally to locate at the region that lies from the blastocoel floor to the dorsal lip at the onset of gastrulation. The organizer located at the blastocoel floor contributes to the anterior axial mesoderm including the prechordal plate, and the organizer at the dorsal lip ends up as the posterior axial mesoderm. During the early step of gastrulation, the anterior organizer moves to establish the physical contact with the prospective neuroectoderm through the “subduction and zippering” movements. Subduction makes a trench between the anterior organizer and the prospective neuroectoderm, and the tissues face each other via the trench. Zippering movement, with forming Brachet's cleft, gradually closes the gap to establish the contact between them. The contact is completed at the equator of early gastrulae and it continues throughout the gastrulation. After the contact is established, the dorsal axis is formed posteriorly, but not anteriorly. The model also implies the possibility of constructing a common model of gastrulation among chordate species.     

Evolution and development of the chordates: collagen and pharyngeal cartilage

Chordates evolved a unique body plan within deuterostomes and are considered to share five morphological characters, a muscular postanal tail, a notochord, a dorsal neural tube, an endostyle, and pharyngeal gill slits. The phylum Chordata typically includes three subphyla, Cephalochordata, Vertebrata, and Tunicata, the last showing a chordate body plan only as a larva. Hemichordates, in contrast, have pharyngeal gill slits, an endostyle, and a postanal tail but appear to lack a notochord and dorsal neural tube. Because hemichordates are the sister group of echinoderms, the morphological features shared with the chordates must have been present in the deuterostome ancestor. No extant echinoderms share any of the chordate features, so presumably they have lost these structures evolutionarily. We review the development of chordate characters in hemichordates and present new data characterizing the pharyngeal gill slits and their cartilaginous gill bars. We show that hemichordate gill bars contain collagen and proteoglycans but are acellular. Hemichordates and cephalochordates, or lancelets, show strong similarities in their gill bars, suggesting that an acellular cartilage may have preceded cellular cartilage in deuterostomes. Our evidence suggests that the deuterostome ancestor was a benthic worm with gill slits and acellular gill cartilages.     

The 724 family of DNA sequences is interspersed about the pericentromeric regions of human acrocentric chromosomes

We describe the organization of the complex, interspersed 724 family of DNA sequences that is distributed in multiple copies about the pericentromeric region of human acrocentric chromosomes. 724 family members were isolated using an efficient recombination-based assay for nucleotide sequence homology to screen a human genomic library. Eight related but distinct 724 family members were isolated that hybridized to a total of 20 different human-genomic EcoRI DNA fragments spanning 100,000 base pairs. In contrast with tandemly clustered satellite and ribosomal DNA sequences also located on the short arms of human acrocentric chromosomes, 724 family members are interspersed. No evidence for local interspersion or homology between 724 family members and ribosomal or satellite DNA sequences was found. Juxtaposition of the complex 724 family to the nucleolus organizer region was a recent event in primate evolution. The unique organization of 724 family members on each of the five human acrocentric chromosomes indicates that the 724 family continues to evolve within the human karyotype.     

Did the first chordates organize without the organizer?

Models of vertebrate development frequently portray the organizer as acting on a largely unpatterned embryo to induce major components of the body plan, such as the neural plate and somites. Recent experiments examining the molecular and genetic basis of major inductive events of vertebrate embryogenesis force a re-examination of this view. These newer observations, along with a proposed revised fate map for the frog Xenopus laevis, suggest a possible reconciliation between the seemingly disparate mechanisms present in the ontogeny of the common chordate body plan of vertebrate and invertebrate chordates. Here, we review data from vertebrates and from an ascidian urochordate and propose that the organizer was not present at the base of the chordate lineage, but could have been a later innovation in the lineage leading to vertebrates, where its role was more permissive than instructive.     

Asexual propagation and regeneration in colonial ascidians

Regeneration is widely distributed among the metazoans. However, clear differences exist as to the degree of regenerative capacity: some phyla can only replace missing body parts, whereas others can generate entirely new individuals. Ascidians are animals that possess a remarkable regenerative plasticity and exhibit a great diversity of mechanisms for asexual propagation and survival. They are marine invertebrate members of the subphylum Tunicata and represent modern-day descendants of the chordate ancestor; in their tadpole stage they exhibit a chordate body plan that is resorbed during metamorphosis. Solitary species grow into an adult that can reach several centimeters in length, whereas colonial species grow by asexual propagation, creating a colony of genetically identical individuals. In this review, we present an overview of the biology of colonial ascidians as a paradigm for study in stem cell and regenerative biology. Focusing on botryllid ascidians, we introduce the potential roles played by multipotent epithelia and multipotent/pluripotent stem cells as source of asexual propagation and regenerative plasticity in the different budding mechanisms, and consider the putative mechanism of body repatterning in a non-embryonic scenario. We also discuss the involvement of intra-colony homeostatic processes in regulating budding potential, and the functional link between allorecognition, chimerism, and regenerative potential.     

Anterior patterning by synergistic activity of the early gastrula organizer and the anterior germ layer tissues of the mouse embryo.

Fragments of the germ layer tissues isolated from the early-primitive-streak (early-streak) stage mouse embryos were tested for axis induction activity by transplantation to late-gastrula (late-streak to early-bud) stage host embryos. The posterior epiblast fragment that contains the early gastrula organizer was able to recruit the host tissues to form an ectopic axis. However, the most anterior neural gene that was expressed in the ectopic axis was Krox20 that marks parts of the hindbrain, but markers of the mid- and forebrain (Otx2 and En1) were not expressed. Anterior visceral endoderm or the anterior epiblast alone did not induce any ectopic neural tissue. However, when these two anterior germ layer tissues were transplanted together, they can induce the formation of ectopic host-derived neural tissues but these tissues rarely expressed anterior neural genes and did not show any organization of an ectopic axis. Therefore, although the anterior endoderm and epiblast together may display some inductive activity, they do not act like a classical organizer. Induction of the anterior neural genes in the ectopic axis was achieved only when a combination of the posterior epiblast fragment, anterior visceral endoderm and the anterior epiblast was transplanted to the host embryo. The formation of anterior neural structures therefore requires the synergistic interaction of the early gastrula organizer and anterior germ layer tissues.     

Early development of coelomic structures in an echinoderm larva and a similarity with coelomic structures in a chordate embryo

Early coelomic development in the abbreviated development of the sea urchin Holopneustes purpurescens is described and then used in a comparison with coelomic development in chordate embryos to support homology between a single arm of the five-armed radial body plan of an echinoderm and the single bilateral axis of a chordate. The homology depends on a positional similarity between the origin of the hydrocoele in echinoderm development and the origin of the notochord in chordate development, and a positional similarity between the respective origins of the coelomic mesoderm and chordate mesoderm in echinoderm and chordate development. The hydrocoele is homologous with the notochord and the secondary podia are homologous with the somites. The homology between a single echinoderm arm and the chordate axis becomes clear when the aboral to oral growth from the archenteron in the echinoderm larva is turned anteriorly, more in line with the anterior–posterior axis of the early zygote. A dorsoventral axis inversion in chordates is not required in the proposed homology.