The Spemann Organizer ofXenopusIs Patterned along Its Anteroposterior Axis at the Earliest Gastrula Stage

AbstractThe Spemann organizer is largely responsible for organizing and patterning the anteroposterior axis during the development of amphibians. In this report, we examine the degree of anteroposterior pattern in the earliest gastrula organizer ofXenopususing a combination of embryological and molecular techniques. When we divide the earliest gastrula organizer, a region measuring 20 cells high by 25 cells wide, into stereotyped anterior (vegetal) and posterior (animal) halves, each half not only has a distinct fate and state of specification, but also induces a unique set of region-specific neural genes. When wrapped in animal cap ectoderm, the anterior half induces only anterior-specific genes (XAG-1andotxA), while the posterior half induces anterior (otxAand reduced levels ofXAG-1) and posterior (Hox B9) neural genes, revealing early localization of neural posteriorizing activity to posterior mesendoderm. This is the earliest demonstration of regionalized neural induction by theXenopusorganizer. Additionally, based on the expression ofgsc, Xbra,andXnot,we show that the organizer is patterned both at the early gastrula stage and prior to the appearance of bottle cells