CSB Special Seminar – Professor Ray Keller, Department of Biology, University of Virginia

Considerable indirect evidence supports the notion that blastopore closure, a major feature of gastrulation in anamniote and a few amniote embryos, involves circumblastoporal, tensile forces generated by the marginal zone, an annular region of largely presumptive mesoderm surrounding the blastopore, but there has been little description of these forces.  For this reason and because blastopore closure was often considered a “non-specific event” because many and varied genetic, molecular, cellular, and tissue manipulations result in its failure, we set about measuring these forces.   Direct measurements of marginal zone explants revealed large, long-range tensile force spanning their entire circumblastoporal extent and generated by Convergent Extension (CE), as expected, but also revealed unexpected forces generated by a another process, Convergent Thickening (CT), before the onset of CE, and throughout the marginal zone, rather than just ventrally as previously thought. Further analysis revealed a progressive transition from CT- to CE-generated force, co-incident with transition to the mediolateral cell intercalation behavior (MIB) underlying CE, and also a strong dependence of the function of local force-generating processes on large-scale mechanical context, specifically tissue stiffness and geometry.  These studies, along with those of others, highlight the fact that morphogenesis is “systems mechanics” process, whose specificity arises from integration of distributed information of several kinds at multiple levels.