The Cellular Basis of Gastrulation in Xenopus laevis: Active, Postinvolution Convergence and Extension by Mediolateral Interdigitation

Time-lapse videomicrographic and SEM analyses of normal and microsurgically altered gastrulation show that the morphogenetic movements of the dorsal marginal zone (DMZ)—extension, convergence, and involution—all result from behavior that occurs after the marginal zone has involuted. Before its involution, the DMZ shows no detectable capacity for autonomous convergence or extension. If its involution is prevented, the DMZ will show convergence and extension but only at developmental stages at or beyond the stage at which it normally would have involuted. Thus autonomous convergence and extension, which have been ascribed to the DMZ are, in fact, properties of the dorsal mesodermal mantle (DMM) and the archenteron roof. SEM analysis of cell shape and packing patterns, suggest that cells of the DMM merge (interdigitate) mediolaterally, between one another, beginning just beyond the point of involution. This behavior is thought to reduce the width and increase the length (postinvolution convergence and extension) of the DMM. The decrease in circumference (width) at the vegetal-most part of the newly involuted DMM forms a constriction ring just inside the blastopore. Constriction and concurrent elongation of the DMM act in concert to move the blastoporal lip vegetally. The DMZ is passively pulled vegetally and over the blastoporal lip as deep cells are recruited for participation in mediolateral interdigitation at the vegetal end of the DMM.