Consistent left-right asymmetry cannot be established by late organizers in Xenopus unless the late organizer is a conjoined twin.

How embryos consistently orient asymmetries of the left-right (LR) axis is an intriguing question, as no macroscopic environmental cues reliably distinguish left from right. Especially unclear are the events coordinating LR patterning with the establishment of the dorsoventral (DV) axes and midline determination in early embryos. In frog embryos, consistent physiological and molecular asymmetries manifest by the second cell cleavage; however, models based on extracellular fluid flow at the node predict correct de novo asymmetry orientation during neurulation. We addressed these issues in Xenopus embryos by manipulating the timing and location of dorsal organizer induction: the primary dorsal organizer was ablated by UV irradiation, and a new organizer was induced at various locations, either early, by mechanical rotation, or late, by injection of lithium chloride (at 32 cells) or of the transcription factor XSiamois (which functions after mid-blastula transition). These embryos were then analyzed for the position of three asymmetric organs. Whereas organizers rescued before cleavage properly oriented the LR axis 90% of the time, organizers induced in any position at any time after the 32-cell stage exhibited randomized laterality. Late organizers were unable to correctly orient the LR axis even when placed back in their endogenous location. Strikingly, conjoined twins produced by late induction of ectopic organizers did have normal asymmetry. These data reveal that although correct LR orientation must occur no later than early cleavage stages in singleton embryos, a novel instructive influence from an early organizer can impose normal asymmetry upon late organizers in the same cell field.

The role of the Spemann organizer in anterior-posterior patterning of the trunk.

The formation of the vertebrate body axis during gastrulation strongly depends on a dorsal signaling centre, the Spemann organizer as it is called in amphibians. This organizer affects embryonic development by self-differentiation, regulation of morphogenesis and secretion of inducing signals. Whereas many molecular signals and mechanisms of the organizer have been clarified, its function in anterior-posterior pattern formation remains unclear. We dissected the organizer functions by generally blocking organizer formation and then restoring a single function. In experiments using a dominant inhibitory BMP receptor construct (tBr) we find evidence that neural activation by antagonism of the BMP pathway is the organizer function that enables the establishment of a detailed anterior-posterior pattern along the trunk. Conversely, the exclusive inhibition of neural activation by expressing a constitutive active BMP receptor (hAlk-6) in the ectoderm prohibits the establishment of an anterior-posterior pattern, even though the organizer itself is still intact. Thus, apart from the formerly described separation into a head and a trunk/tail organizer, the organizer does not deliver positional information for anterior-posterior patterning. Rather, by inducing neurectoderm, it makes ectodermal cells competent to receive patterning signals from the non-organizer mesoderm and thereby enable the formation of a complete and stable AP pattern along the trunk.