Analysis of Biological Development (K. Kalthoff)

Updates to Topic 10: Gastrulation


Answers to Questions in Text

Archenteron Elongation in Sea Urchn Embryos (p. 229/230)

  1. In Fig. 10.8a, the convergent extension of the archenteron by means of coordinated cell shape changes is drawn in a way that involves a decrease in the archenteron's diameter. Could there be a convergent extension by coordinated cell shape change that leads to an elongation of the archenteron without a concomitant decrease in diameter? Answer: Yes, the archenteron could grow in length while maintaining its diameter if the thickness of the archenteron wall decreased. In other words, individual cells could increase in length while maintaining width and losing radial thickness. This was indeed observed, especially in one species, Lytechinus pictus.
  2. Convergent extension, whether it is driven by coordinated cell shape change or by cell intercalation, is thought to be an active movement, that is, driven by forces acting within the archenteron itself. Could the elongation of the archenteron also be a passive movement, driven by forces outside the archenteron? How could this proposition be tested? Answer: In principle, one would have to keep the isolated archenteron in culture and observe whether it will undergo convergent extension on its own. If it does, then the convergent extension movement must be active. A negative result would be less conclusive. In this case, one would need to test more specific hypotheses about the nature of the external force and eliminate it selectively in an otherwise intact embryo. Traction by secondary mesenchyme cells is such an external force, as discussed subsequently in the text.

Cell Intercalation in Involuting Marginal Zone of Xenopus Embryo (p. 238)

  1. 1. Do you expect the extensive cell intercalations shown in Fig. 10.18c to occur mainly in the deep layer or in the superficial layer of the involuting marginal zone? How would you test your hypothesis experimentally? Answer: In the deep layer of the involuting marginal zone. Test by transplanting only labeled D-IMZ or S-IMZ.
  2. Based on the results shown in Fig. 10.11 h and i, do you expect the extensive cell intercalations shown in Fig. 10.18c to occur on the ventral side of the embryo as well? Answer: No, since there is little if any convergent extension occurring in the ventral involuting marginal zone no cell intercalation is expected in this area.

Comments

Clarifications and Corrections

p. 225, left column, line 6. The web address should read "http://www.esb.utexas.edu/kalthoff/bio349"

p. 238, left column, last paragraph (shaded), line 6, should read: "between equivalent regions of the embryo,..."

New Review Articles

Wallingford J.B., Fraser S.E. and Harland R.M. (2002) Convergent extension: the molecular control of polarized cell movement during embryonic development. Devel. Cell 2: 695-706

Peifer M. and McEwen D.G. (2002) The ballet of morphogenesis: Unveiling the hidden choreographers. Cell 109: 271-274

New Research Articles

Dijane A., Riou J.-F., Umbhauer M., Boucaut J.-C. and Shi D.-L. (2000) Role of frizzled7 in the regulation of convergent extension movements during gastrulation in Xenopus laevis. Development 127: 3091-3100

Tada M. and Smith J.C. (2000) Xwnt11 is a target of Xenopus Brachyury: regulation of gastrulation movements via Dishevelled, but not through the canonical Wnt pathway. Development 127: 2227-2238

Wallingford J.B., Rowning B.A., Vogeli K.M., Rothbacher U., Fraser S.E. and Harland RM. (2000) Dishevelled controls cell polarity during Xenopus gastrulation. Nature 405: 81-85

Wallingford J.B., Goto T., Keller R., Harland R.M. (2002) Cloning and expression of Xenopus Prickle, an orthologue of a Drosophila planar cell polarity gene. Mech. Dev. 116: 183-186

The four studies quoted above address the basic problem of how morphogenetic movements are genetically controlled. The focus is on the convergent extension movement during gastrulation in Xenopus. Tada and Smith found that this movement depends on the functions of Xbra+, Xwnt11+, and Xdsh+. These genes encode Brachyury (a transcription factor), Wnt11 (a secreted signal protein), and Dishevelled (a transducer of the Xwnt signal), respectively. The Xenopus Dishevelled+ (Xdsh+) gene is named after its Drosophila ortholog Disheveled+, which is required for the regular formation of wing hairs. Wallingford et al. (2000) interfered with Xdsh function by injecting blastomeres with mRNA translated into excess amounts of normal Xdsh protein (overexpression) or defective Xdsh protein (dominant negative approach). In either case, they found that mesoderm failed to undergo convergent extension and lost mediolateral cell polarity. Specifically, the disturbed cells do not restrict the extension of lamellipodia to the mediolateral dimension as they do in normal embryos (see Figs. 1 and 2 below). The localization of Xdsh at the membrane of normal dorsal mesodermal cells also suggests that Xdsh may control cell polarity, especially since in Drosophila wing cells the Dishevelled protein is concentrated where the wing hair is formed. Wallingford et al. (2002) show that Xenopus Prickle+ (XPk+), an ortholog of the Drosophila planar cell polarity gene Prickle+, is expressed in tissues at the dorsal midline during gastrulation and early neurulation. Djiane et al. observed that overexpression of Xenopus frizzled7+ (Xfz7+), which encodes a receptor of Xwnt11, also disturbs convergent extension. The effect of overexpressing wild-type Xfz7+ is counteracted by the synthesis of a defective (soluble) version of the same receptor, which binds up Xwnt11 without causing a biological response. (See Topic 15, pp. 393-395, for a general discussion of "dominant interference" strategies.) Together, these studies identify a well-conserved molecular pathway that controls planar cell polarity which, in the case of Xenopus, is critical to the morphogenetic movement of convergent extension.

Fig. 1: Convergent extension of Xenopus mesoderm (IMZ-D).

(A) The cells shown are in the middle of the prospective notochpord or a somite. Before gastrulation (to the left), cells extend and retract lamellipodia (red) randomly. As gastrulation begins, cells become poilarized, stabilizing lamellipodia medially and laterally. The stabilized lamellipodia extert traction on neighboring cells, causing mediolateral intercalation, and thus, convergent extension.

(B) Cells located near the notochord-somite boundary display the phenomenon of boundary capture. Cells remain at the boundary and extend lamellipodia only at the site opposite to the attachment. The result is again mediolateral intercalation and

Fig.2: Dependence of planar cell polarity of the Dishevelled protein in Drosophila and Xenopus.

(A) Epithelial cell in a Drosophila wing. Dishevelled protein (green) is localized inside the plasma membrane and concentrated near the site where a hair is formed. In wild-type wings, the wing hairs look very regular because all cells are polarized in the same direction. In loss-of-function alleles of Dishevelled, planar cell polarity is lost.

(B) Xenopus involuting marginal zone (IMZ-S). Xenopus Dishevelled (Xdsh) protein is also localized along the plasma membrane, but concentration near the sites of lamellipodium formation has not been shown yet.

(C, D) An excess of both wild-type as well as dominant-negative Xdsh interferes with planar cell polarity.


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