Analysis of Biological Development (K. Kalthoff)

Updates to Topic 12: Neurulation and Neural Induction

Answers to Questions in Text

Organizer Experiment (p. 291/292)

  1. Could the lack of a head in the secondary embryo shown in Fig. 12.17 be related to the fact that the graft had come from a late gastrula donor? Explain. Answer: Yes, the dorsal blastopore lip had already begun to involute in the donor and therefore may have not been included in the graft. If so, the missing portion of the axis should be expected to be the anterior one, which depends on the portion of the blastopore lip that involutes first and moves the furthest anterior.
  2. The notoplate of the secondary embryo shown in Fig. 12.17 is obviously derived from the graft. Can this be reconciled with the view that the involuting marginal zone does not contribute to ectoderm? If so, which experimental parameters may decide whether the notoplate of the secondary embryo is contributed by the host or by the graft? Answer: The graft may have inadvertently included part of the prospective notoplate because the latter is directly adjacent to the dorsal blastopore lip (involuting marginal zone), which Spemann and Mangold were aiming to transplant. Because the prospective notoplate undergoes a dramatic convergent extention (see Fig. 12.10), a narrow transverse strip of non-involuting marginal zone in the graft will suffice to produce the long stretch of notoplate seen in Fig. 12.17. However, fate mapping studies in chicken indicate that Hensen's node, the chicken equivalent of Spemann's organizer, does itself contribute to the notoplate (see Fig. 10.28).

Planar Induction of Gene Expression Pattern (p. 297/298)

  1. The results shown in Fig. 12.24 were obtained with in situ hydridization probes to three different mRNAs. Can several such probes be applied simultaneously to the same tissue section or embryo? Answer: Yes, because each probe will bind specifically to complementary RNA sequences in the fixed tissue. However, a mixed probe may produce a greater background of unspecifically bound probe than a single probe.
  2. As an alternative to in situ hybridization, what other method could the investigators have used to compare patterns of region-specific gene expression in embryos versus Keller sandwiches? Answer: Immunostaining of the cognate protein products (see Method 4.1).
  3. This stunning results reported by Doniach and coworkers could be ascribed to an unnoticed planar migration of mesoderm cells from deep involuting marginal zone into the adjacent non-involuting marginal zone and a subsequent response of the “invaded” non-involuting marginal region to vertical signals from the putative migratory cells. How could the investigators have tested (and they did!) whether such a planar cell migration did indeed occur? Answer: By forming composite Keller sandwiches consisting of non-involuting marginal zone from unlabeled donors stuck in its natural position against involuting marginal zone from labeled donors. At intervals, they tested for labeled invader cells in the unlabeled zone. None were detected.


Clarifications and Corrections

p. 281, Fig. 12.3a: the second label should read "Neural plate".

p. 297, Fig. 12.23 a,b: the blue and tan colors represents the superficial and deep layers of the marginal zone and animal cap. Involution has not occurred yet at this early gastrula stage.

p.300, Fig. 12.26: In the pale blue box to the lower left, replace the label "Ventral development" with "Dorsal development inhibited". The two labels are equivalent in terms of the proposed model, but the new label is more consistent with the inhibitory action of BMP-4.

New Review Articles

De Robertis EM, Wessely O, Oelgeschlager M, Brizuela B, Pera E, Larrain J, Abreu J, Bachiller D. (2001) Molecular mechanisms of cell-cell signaling by the Spemann-Mangold organizer. Int J Dev Biol. 45: 189-197

Gerhart J. (2001) Evolution of the organizer and the chordate body plan. Int J Dev Biol. 45: 133-153

Harland R. (2000) Neural induction. Curr Opin Genet Dev. 10: 357-362

Joubin K. and Stern C. D. (2001) Formation and maintenance of the organizer among the vertebrates. Int J Dev Biol. 45: 165-175

New Research Articles

Elul T. and Keller R. (2000) Monopolar protrusive activity: A new morphogenetic cell behavior in the neural plate dependent on vertical interactions with the mesoderm in Xenopus. Devel. Biol. 224: 3-19

Extending earlier work on the patterning of cellular behaviors driving morphogenetic movements, the investigators videorecorded explants from Xenopus neurulae of neural tissue with and without underlying mesoderm. With mesoderm, notoplate cells show randomly oriented motility while other neural plate cells show monopolar medially directed motility. Without underlying mesoderm, neural plate cells do not form a notoplate, and all cells express bipolar mediolateral motility. All these behaviors develop in an anterior-to-posterior and lateral-to-medial progression. These results show that morphogenetic behaviors of neural plate cells depend on vertical signals from underlying mesoderm.

Nishita M., Hashimoto M.K., Ogata S., Laurent M.N., Ueno N., Shibuya H. and Cho K.W.Y. (2000) Interaction between Wnt and TGF- signalling pathways duringformation of Spemann's organizer. Nature 403: 781-785

In the amphibian gastrula, the Wnt and TGF- signalling pathways are both required for the establishment of Spemann's organizer. The investigators show, by co-immunoprecipitation, that -catenin and other downstream components of the Wnt cascade form a complex with Smad4, a mediator of TGF- signals. The interaction directly and synergistically affects expression of an organizer-specific gene, twin+.

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