A main achievement of gastrulation

Gastrulation is the morphogenetic process by which, in metazoans, the germ layers are established in their characteristic spatial arrangement: ectoderm on the outside, endoderm inside, and mesoderm, if present, in between these two layers. Most often, the main activity of gastrulation involves the movement of the endoderm and mesoderm from the surface of the embryo to the interior, and a variety of mechanisms has developed in different groups of animals to achieve this internalization of germ layers. Gastrulation movements have been extensively studied in amphibians, notably in Xenopus. In the amphibian blastula, the wall enclosing the blastocoel consists of a thin animal blastocoel roof (BCR) and a massive vegetal region. Most of the BCR will form ectoderm. During gastrulation, it spreads in the process of epiboly to eventually cover the embryo. The vegetal cell mass will contribute to the endoderm, and between vegetal region and BCR, in the marginal zone, mesoderm surrounds the embryo as a belt below the equator (Vogt, 1929; Keller, 1986). In Xenopus, the outer part of the dorsal marginal zone consists of prospective axial mesoderm, whereas the deep zone includes the head mesoderm. The endoderm extends into the marginal zone in the superficial layer, thus largely covering the mesoderm from the outside (Keller, 1975, 1976; Minsuk and Keller, 1997). Nevertheless, mesoderm is initially part of the blastocoel wall, and must be internalized during gastrulation together with the endoderm. Despite the fundamental importance of this process, its mechanism is not well understood in Xenopus. Blastopore indentation at the vegetal boundary of the mesodermal belt is initiated by bottle cell formation, first dorsally and then laterally and ventrally (Hardin and Keller, 1988). However, the blastopore groove formed is shallow. Its deepening and the concomitant internalization of the mesoderm occur by involution: mesoderm and suprablastoporal endoderm roll over the blastopore lip, thereby turning inward. This is seen directly in time-lapse recordings (Keller, 1978), but is also evident from the fate map. In the early gastrula, more anterior mesoderm is located closer to the blastopore, more posterior mesoderm further animally, suggesting a rotation of the mesoderm in the course of gastrulation that inverts the anterior-posterior axis (Keller, 1976). It is generally assumed that the force for involution is provided by the mesoderm, whereas the mass of yolk-rich vegetal cells is internalized passively, e.g. by constricting the ring-like blastopore below the vegetal region. In fact, however, the driving force for mesendoderm internalization has not yet been localized experimentally. We show that contrary to previous belief, it is indeed an active movement of the vegetal cell mass that promotes initial mesoderm involution and the inward surging of prospective endoderm.