In light of recent phylogenetic analyses placing cephalochordates basally in the chordate lineage, we propose that separate signalling centres for patterning the D/V and A/P axes may be an ancestral chordate character. These results suggest evolutionary conservation of the mechanisms for both D/V and A/P patterning of the early gastrula. In addition, Wnt-antagonists (for example, Dkks and sFRP2-like) are expressed anteriorly, whereas Wnt genes themselves are expressed posteriorly, consistent with a role for Wnt signalling in anterior/posterior (A/P) patterning. Here we show that the gastrula of the cephalochordate amphioxus expresses dorsal/ventral (D/V) patterning genes (for example, bone morphogenetic proteins ( BMPs), Nodal and their antagonists) in patterns reminiscent of those of their vertebrate orthlogues, and that amphioxus embryos, like those of vertebrates, are ventralized by exogenous BMP protein. Although a topic of long-standing interest, the evolutionary origin of the organizer remains unclear. We then constructed a cDNA library from this mRNA, which is presumably enriched in molecules encoding proteins that provide positional information specific for the organizer, and screened for molecules that are likely to be involved in conferring organizer activity.The organizer of the vertebrate gastrula is an important signalling centre that induces and patterns dorsal axial structures. We investigated the molecular basis of the organizer by isolating mRNA from excised dorsal lips (Fig. Thus, it can be inferred that the inductive potential of the organizer is dependent on both regional specialization in, and the overall size of, the dorsal blastopore lip. The quantity of organizer tissue in a Xenopus laevis embryo is directly proportional to the extent of anterior development. ![]() The types of tissues formed by each fragment depended on its original location along the longitudinal axis in the dorsal lip. That localized positional information exists in the organizer is indicated by experiments in which the salamander dorsal lip was divided into a number of fragments and cultured separately. Īlthough single growth factors may induce a variety of mesodermal structures and even confer organizer-like activity on uncommitted ectoderm or microinjected blastomeres, the organizer's actual mode of action is likely to involve a number of molecules that jointly provide the spectrum of activities necessary for the induction of a body axis. Therefore, it has been postulated that mesoderm-inducing growth factors may provide positional information along the anterior-posterior axis by the regulation of the expression of homeobox genes. However, these growth factors have been shown to influence the expression of several position-specific homeobox genes. The exact relation between mesoderm induction and the organizer phenomenon is not clear at present. Although it is not known whether any of these molecules is a natural inducer in vivo, they are believed to be, or be related to, the actual molecules involved in mesoderm induction. Activin protein can organize body axis in uncommitted ectoderm and activin mRNA, injected into a single vegetal blastomere, can organize a secondary body axis. Įxperiments have shown that peptide growth factors related to transforming growth factor- (TGF, XTC-MIF, activin) and basic fibroblast growth factor can induce mesoderm formation in uncommitted ectoderm and even confer organizer activity on treated ectoderm in transplantation experiments. The biochemical basis of the organizer phenomenon has, however, remained elusive, despite intensive investigation. The anterior-posterior extent of the secondary axis induced by the transplanted dorsal lips differed as gastrulation proceeded dorsal lips from early gastrulae could induce nearly complete axes including heads, whereas dorsal lips from late gastrulae induced axes consisting of trunk and tail. ![]() ![]() The dorsal blastopore lip was called the "organizer" to reflect its ability to recruit or organize host cells to form a secondary axis with appropriate anterior-posterior and dorsal-ventral polarity. THE DORSAL BLASTOPORE LIP FROM an early salamander gastrula, when implanted into the ventral side of a recipient gastrula, can organize the formation of a secondary body axis consisting of both host and graft-derived tissue.
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