The dorsal lip refers to the section of tissue located at the site of the first invagination in the developing pregastula and is understood to act as both the neural inducer in the early embryo as well as the overall organizer of the entire body axis. One such candidate gene, noggin, is sufficient for rescuing development of irradiated embryos when its mRNA is injected (C). Establishment of cDNA libraries from extracted dorsal lip mRNA identified candidate genes that may be responsible for neural induction. Injection of extracted mRNA from the dorsal lip into irradiated Xenopus embryos rescued neural induction and overall development demonstrating there is a genetic basis of neural induction (B). Figure 1: Mangold's dorsal lip transplant experiment in Xenopus demonstrated that a transplanted dorsal lip could induce the formation of a double axis in the new host embryo, solidifying the dorsal lip's sufficiency in neural induction (A). It is particularly important for its role in neural induction through the default model, where signaling from the dorsal lip protects a region of the epiblast from becoming epidermis, thus allowing it to develop to its default neural tissue. The dorsal lip is formed during early gastrulation as folding of tissue along the involuting marginal zone of the blastocoel forms an opening known as the blastopore. 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 dorsal lip of the blastopore is a structure that forms during early embryonic development and is important for its role in organizing the germ layers. 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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