Posterior Neural Plate Research Articles

Inductive Events in the Patterning of the Xenopus laevis Hatching and Cement Glands, Two Cell Types Which Delimit Head Boundaries

In Xenopus, the hatching and cement glands form at important boundaries in the head. We have examined induction of these glands in order to see the basis for this ectodermal patterning. Ectoderm can be induced to form hatching and cement glands by dorsal mesoderm and by the neural plate. Because the neural plate is sufficient to induce hatching and cement glands and lies adjacent to presumptive hatching and cement glands at the time of induction, it is the best candidate for the in vivo inducer of these tissues. The cement gland is restricted to the front of the head in part because the anterior but not posterior neural plate is capable of inducing it. The hatching gland is also restricted to the head, but can be induced by both anterior and posterior neural plates. Therefore, some factor suppresses hatching gland differentiation in the trunk. Transplanted neural plate pieces induced hatching gland cells in the ectoderm of embryos, suggesting that the inductive signal is planar. Treatment with retinoic acid or lithium at the start of gastrulation caused a loss of head structures. The presence of hatching gland in lithium-treated embryos suggests that these two agents have distinct effects and supports the idea that the induction of hatching and cement glands involves different pathways.

Ectopic expression of the proto-oncogene int-1 in Xenopus embryos leads to duplication of the embryonic axis

While there is convincing evidence implicating Drosophila int-1 in pattern regulation, the normal role of int-1 in vertebrate development is unclear. We have injected Xenopus eggs with mouse int-1 RNA and monitored subsequent development. Injected RNA is translated and the protein widely distributed. Embryos develop into apparently normal gastrulae, but almost all surviving neurulae have a bifurcated anterior and expanded posterior neural plate. Bifurcation of the neural plate was abolished by substitution of a single, conserved cysteine residue and was dependent on the presence of a signal peptide sequence in the int-1 protein. Histological examination indicates that underlying axial mesodermal structures were duplicated. This result suggests that ectopic int-1 expression leads to dual axis formation and points to a role for int-1 in patterning processes in vertebrate development.

Mesoderm induction in the future tail region of Xenopus.

We have used interspecific grafts between Xenopus borealis and Xenopus laevis to study the signalling system that produces tail mesoderm. Early gastrula ectoderm grafted into the posterior neural plate region of neurulae responds to a mesodermal inducing signal in this region and forms mainly tail somites; this signal persists until at least the early tail bud stage. Ventral ectoderm grafted into the posterior neural plate loses its competence to respond to this signal after stage 10 1/2. We have established the specification of anterior and posterior neural plate ectoderm. In ectodermal sandwiches or when grafted into unusual positions, anterior regions gave rise to mainly nervous system and posterior regions to large amounts of muscle, together with some nervous system. Thus it was impossible to assess the competence of posterior neural plate ectoderm to form further mesoderm and hence to establish if mesodermal induction continues during neurulation in unmanipulated embryos.

Analysis of embryonic sources and mechanims of development of posterior levels of chick neural tubes.

AbstractThe mechanisms of development of posterior levels of neural tubes of chick embryos were analyzed by study of serial cross‐sections of a continuous series of normal embryos between 40 to 72 hours of incubation. Two extirpation experiments were performed in ovo on other embryos of the same stages. Descriptive studies revealed the presence of an overlap zone in which two types of neural tube formation occurred. Open neural tube formation (by fusion of neural folds) occurred dorsally in this region; closed neural tube formation (by canalization of solid medullary cord tissue) occurred ventrally. Extirpation of the posterior end of the neural plate produced defects within the lumbosacral region, indicating that the posterior neural plate participates in the formation of the lumbosacrum, and that the overlap zone is therefore in the lumbosacral region. Extirpation of the prospective neural tissue in the anterior end of the tail bud indicated that only the most posterior levels of the neural tube originate exclusively by cavitation of the tail bud. In both extirpation experiments a neural tube formed independently within the tail bud tissue, indicating that formation of the neural tube in this region is not dependent upon direct continuity with neural tissue anteriorly.

Effect of Protein Extracts of Neural Plate Plus Chordamesoderm on Presumptive Epidermis.

SummaryProtein extracts of the neural plate plus underlying chordamesoderm induce the presumptive epidermis of the gastrula to form neural tubes. However no differences were obtained in the structure of the neural tubes which were induced by extracts of anterior as compared with posterior regions of the neural plate and underlying chordamesoderm. Preparations of the anterior and posterior regions of the neural plate and underlying chordamesoderm which were killed by freezing and subsequent drying induced neural tubes in the explanted presumptive epidermis of the gastrula. Quantitative differences in the frequency of induction were obtained with the posterior neural plate and underlying chordamesoderm inducing with the greatest frequency.