Lithium dorsalizes but also mechanically disrupts gastrulation of Xenopus laevis

The discovery that lithium treatment at blastula stages can induce axis formation suggested that it might act by respecifying the cytoplasmic rearrangement-generated dorsoventral pattern, so that ventral cells behave like their dorsal counterparts. We have studied the effects of Li+ treatment on the spatial layout of the cell-group movements of gastrulation to see whether this is the case. We find that involution of the chordamesoderm and associated archenteron roof is retarded by Li+, an effect which does not suggest dorsal respecification. However, in both migration of the leading edge mesoderm and convergent extension of the marginal zone, ventral regions clearly do show dorsal-type movement. Because of this, and because of examples where disruption of involution and effects on axis differentiation do not correlate, we propose that failure of involution represents a distinct effect of Li+ involving disruption of mechanical relationships at the blastopore. Thus archenteron formation poorly reflects the dorsoventral pattern. Extension of sandwich explants of the ventral marginal zone is proposed as a reliable quantitative assay for alterations to the dorsoventral pattern.
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