Morphological polarity of intercalating deep mesodermal cells in the organzer of Xenopus laevis gastrulae

The dorsal lip of the blastopore constitutes the “organizer” of the amphibian body plan, both in terms of its tissue interactions and its morphogenetic movements of convergence and extension during gastrulation. This tissue autonomously narrows (converges) and lengthens (extends) during early development, functioning prominently in the morphogenetic movements of both gastrulation and neurulation Xenopus laevis. High resolution time-lapse recording of cell behavior in cultured explants and cell labelling studies have shown that the movements of convergence and extension are produced by radial intercalation of cells, in which several layers rearrange to produce fewer layers of greater area, and by mediolateral intercalation of cells, in which several rows of cells rearrange to produce a narrower, longer array. By labelling individual cells with the fluorescent compound, DiI, and making low light level recordings, we found that cells of the notochord intercalate mediolaterally using polarized protrusive activity at their internal medial or lateral ends. Thus polarized protrusive activity appears to play a major role in mediolateral cell intercalation after the boundary between the notochord and somites forms in the late gastrula stage.We examine further the morphology of the deep mesodermal cells with scanning electron microscopy at earlier stages, to search for morphological manifestations of a similar polarity of protrusive activity. The dorsal deep mesodermal cells of early gastrulae were exposed by rapidly pulling the. epithelial endoderm off the deep cells with forceps, in Danilchik's solution, and fixing the embryo within 15 seconds in 2% glutaraldehyde in 0.1 M sodium cacodylate buffer (pH 7.4) as described previously. The dorsal deep cells adjacent to the epithelium are elongate and aligned parallel to one another and to the circumference of the glastopore (Fig. 1). The medial and lateral ends bear broad, lamelliform protrusions (large pointers, Fig. 1, 2) whereas the anterior and posterior ends bear numerous small filiform protrusions (small pointers, Fig. 1, 2). This characteristic morphology is found only in the dorsal marginal zone, which undergoes convergence and extension by mediolateral intercalation.