Induction of notochord by the organizer inXenopus.

One important step in understanding early development is to define the cell interactions involved in establishing tissue types. In amphibian embryos, one such interaction is the induction by the organizer region after the late blastula stage of lateral and ventral regions of the marginal zone (MZ) to form dorsal tissue types such as muscle. It is not known whether the organizer can also induce lateral MZ to form notochord after the late blastula stage. We find that this induction occurs under experimental conditions and plays a role in normalXenopus development. The ability to induce notochord is strongest at the center of the organizer along the dorsal midline and weaker at the lateral edges of the organizer. Organizer tissue along the dorsal midline, which would differentiate as notochord in normal development, can exhibit organizer functions such as the induction of the dorsolateral MZ to form notochord without later differentiating as notochord itself. Thus organizer activity can be dissociated from subsequent notochord formation.