Regional requirements for Dishevelled signaling duringXenopusgastrulation: separable effects on blastopore closure, mesendoderm internalization and archenteron formation

Abstract

During amphibian gastrulation, the embryo is transformed by the combined actions of several different tissues. Paradoxically, many of these morphogenetic processes can occur autonomously in tissue explants, yet the tissues in intact embryos must interact and be coordinated with one another in order to accomplish the major goals of gastrulation: closure of the blastopore to bring the endoderm and mesoderm fully inside the ectoderm, and generation of the archenteron. Here, we present high-resolution 3D digital datasets of frog gastrulae, and morphometrics that allow simultaneous assessment of the progress of convergent extension, blastopore closure and archenteron formation in a single embryo. To examine how the diverse morphogenetic engines work together to accomplish gastrulation, we combined these tools with time-lapse analysis of gastrulation, and examined both wild-type embryos and embryos in which gastrulation was disrupted by the manipulation of Dishevelled (Xdsh)signaling. Remarkably, although inhibition of Xdsh signaling disrupted both convergent extension and blastopore closure, mesendoderm internalization proceeded very effectively in these embryos. In addition, much of archenteron elongation was found to be independent of Xdsh signaling, especially during the second half of gastrulation. Finally, even in normal embryos, we found a surprising degree of dissociability between the various morphogenetic processes that occur during gastrulation. Together, these data highlight the central role of PCP signaling in governing distinct events of Xenopusgastrulation, and suggest that the loose relationship between morphogenetic processes may have facilitated the evolution of the wide variety of gastrulation mechanisms seen in different amphibian species.

The functions of maternal Dishevelled 2 and 3 in the Early Xenopus embryo

“…In Xenopus, previous studies have implicated Dvl in polarized cell movement during gastrulation and neurulation (Sokol et al, 1995;Sokol, 1996;Wallingford et al, 2000;Wallingford and Harland, 2001;Ewald et al, 2004). Our data provide the first evidence that this function of Dvl in morphogenetic movements at the gastrula and neurula stages is carried out by maternally supplied proteins.…”

“…Xenopus Dvl2 (formally known as Xdsh) has been most extensively studied by overexpression of either the wild type or mutant forms of the protein. These studies suggest an important role for Dvl2 in non-canonical Wnt signaling specifically in the convergence extension movements involved in gastrulation and neurulation (Ewald et al, 2004;Wang et al, 2006). More controversial is the role of Dvls in canonical Wnt signaling.…”

Dynamic determinations: patterning the cell behaviours that close the amphibian blastopore

“…Conversely, an uncoordinated or patchy expression of MIB should result in loss of continuity of hoop stress and failure to build up large-scale, tissue-level convergence forces, and as in the case of experimentally breaking hoop stress continuity, would result in locally independent extensions and failure of blastopore closure and also proper involution (Schechtman 1942;Keller 1981). Involution and internalization can occur without convergent extension (Schechtman 1942;Keller & Danilchik 1988;Ewald et al 2004) but it is a short, shallow, symmetrical slit of an archenteron, very unlike the asymmetric, dorsally elongated one normally produced by involution with convergent extension.…”

Two‐dimensional and three‐dimensional time‐lapse microscopic magnetic resonance imaging of Xenopus gastrulation movements using intrinsic tissue‐specific contrast

“…Thus, approximately 150 m of the archenteron's length at this time point is due to epiboly, while approximately 250 m is due to invagination. This finding is an example for the requirement of a longitudinal study by MRI, as the extension of the archenteron cannot be traced from time course data due to the weak correlation between external staging criteria (blastopore closure) and internal morphogenesis (Ewald et al, 2004).…”

“…Experiments on morphogenetic movements and their molecular control have relied on tissue explants or the interpretation of movements from static histological preparations (Vogt, 1929;Brachet, 1935;Nieuwkoop and Florschü tz, 1950;Keller, 1975Keller, , 1976Nakatsuji, 1975;Bauer et al, 1994;Winklbauer and Schü rfeld, 1999;Davidson et al, 2002;Davidson and Keller, 2004). However, early cleavages are irregular, and it has been found that even the most prominent gastrulation features like archenteron inflation, invagination, and blastopore closure do not occur at invariant times in relation to each other (Ewald et al, 2004). Such properties make the reconstruction of amphibian gastrulation movements from timecourse data problematic, thus highlighting the necessity for time-lapse studies in the same embryo.…”