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.