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Mechanisms, variation, conservation, and integration of early morphogenic machanies in vertebrates

Thursday 27th September 2001 - 10:00 to 11:00
INI Seminar Room 1
Session Title: 
Vertical Integration in Biology: From Molecules to Organisms
Investigation of the mechanism and morphogenic function of convergence, extension, and ingression of cells in the early morphogenesis of several species of amphibians shows that similar morphogenic movements are driven by different cell behaviors and similar cell behaviors have different morphogenic consequences, depending on how the behaviors are integrated in the larger context. Convergence and extension of the axial mesoderm, the paraxial mesoderm and the neural plate play major roles in gastrulation, neurulation, and body axis formation in amphibians, and probably in other vertebrates as well. Convergent extension of both mesodermal and neural tissues in the anuran (tail-less amphibian), Xenopus laevis, share the feature of occurring by mediolateral intercalation of an initially short and wide array of cells to produce a longer, narrower array. These tissues differ in that intercalation of mesodermal cells is driven by a bipolar, mediolaterally oriented protrusive activity whereas intercalation of neural cells is driven by a medially directed, monopolar protrusive activity. They also differ in that the normal, monopolar mode of neural cell intercalation is dependent on the midline tissues of notoplate or notochord, whereas no midline is defined in the bipolar mode of mesodermal cell intercalation. Despite their differences, the biomechanical integration of these local cell intercalation behaviors is similar in the two tissues- a pushing force is exerted in the anterior-posterior axis and tension is exerted in the transverse, mediolateral axis. The result of the pushing forces result in extension of both the mesoderm and neural tissue in the anterior-posterior axis. The mesoderm stiffens in the anterior-posterior axis during extension, thus increasing its resistance to buckling. In contrast, the consequence of the transverse tension generated by convergence is context dependent and differs between neural and mesodermal tissues. The lateral edges of the neural plate are free to move to the midline as the attached, lateral epidermis spreads, allowing neural fold fusion and neural tube closure. In the mesodermal tissue, however, the lateral edges are attached to the contracting vegetal endoderm. As a result, convergence generates hoop stress across the dorsal lip, which pulls the dorsal midline ventrally, thus aiding and abetting involution and blastopore closure. Thus similar cell behaviors produce dramatically different morphogenic results, dependent on their context.
University of Cambridge Research Councils UK
    Clay Mathematics Institute London Mathematical Society NM Rothschild and Sons