The Role of Planar and Early Vertical Signaling in Patterning the Expression ofHoxb-1inXenopus

Abstract

In this paper we examine the contributions of planar and vertical signaling to the patterning of gene expression in neural development and we examine the routes of this neural induction. We have examined how the expression of Xenopus homeobox gene, Hoxb-1, is regulated by instruction from the mesoderm and/or endoderm and ask whether this instruction is by the vertical or planar routes. We investigated normal expression patterns of Hoxb-1 during early Xenopus development and Hoxb-1 expression in sandwich explants of the dorsal marginal zone, which putatively allow only planar signals to pass from the mesodermal and endodermal tissue (Spemann's organizer) to the prospective neural tissue. In the latter case we found significant variability of expression. Observations during dissections suggested that variable degrees of invasion of the mesodermal-endodermal tissue at the leading edge of the mesodermal mantle might be the cause of this variability. Alternatively, differing lengths of time that the prospective neural region spends in planar contact with tissues of the lateral or ventral regions of the embryo could also contribute to this variability. Analysis of staged Keller sandwich explants, "skewered" sandwiches, in which the degree of contact with underlying, involuted mesoderm-endodermal tissues was marked, and "over-the-pole" and "giant" sandwich explants, in which the degree of planar contact with lateral or ventral tissues was normalized, suggests that both planar and vertical signals are involved in induction and patterning of Hoxb-1 expression. The shift in Hoxb-1 expression from a broad, diffuse pattern to a local, focused pattern, characteristic of the ultimate expression pattern in vivo, does not reflect variable degrees of contact with ventral or lateral tissues, but rather reflects early vertical contact with underlying mesodermal-endodermal tissues. We observed such contact at early gastrula stages (stages 10 to 10+), stages commonly assumed not to have the potential for vertical signaling. As the bottle cells first begin to form, at stage 10-, a massive rotation of the lower involuting marginal zone occurs around an internal lip ("levre interne," Nieuwkoop and Florschutz, 1950). This rotation initiates the formation of the Cleft of Brachet from the floor of the blastocoele and brings the prospective mesoderm and endoderm at the leading edge of the marginal zone into vertical apposition with the prospective neural region quite early in gastrulation. The consequence and importance of recognizing these early internal rearrangements are that it pushes backward the time at which potential vertical inductive interactions between mesoderm and neurectoderm can occur. This means that a purely planar inductive situation can cease to exist as early as the inception of bottle cell formation and that neural patterning through vertical induction starts at the very beginning of gastrulation.

Xenopus fibrillin is expressed in the organizer and is the earliest component of matrix at the developing notochord‐somite boundary

“…3H). However, in contrast to intact embryos, no XF expression is seen along the extended neural axis of these explants, which lack a floorplate under these culture conditions (Poznanski and Keller, 1997).…”

Molecular model for force production and transmission during vertebrate gastrulation

“…3A). This device measures the cumulative tensional convergence forces developed by pairs of control or morphant marginal zone explants in a 'giant sandwich' configuration (Sater et al, 1993;Poznanski and Keller, 1997). We found that explants derived from RLC morphant embryos converged with about 500 nN less force than control explants by the end of gastrulation (Fig.…”

PatS and Products of Nitrogen Fixation Control Heterocyst Pattern

“…Pattern formation and cell fate determination have been studied intensively in a number of eukaryotic organisms (7,9,11,12,15). Competition and lateral inhibition among equivalent cell groups are common themes underlying the mechanisms of pattern formation and organogenesis (18).…”