MICROTUBULE DISRUPTION REVEALS THAT SPEMANN ORGANIZER IS SUBDIVIDED INTO 2 DOMAINS BY THE VEGETAL ALIGNMENT ZONE

Mediolateral cell intercalation is proposed to drive morphogenesis of the primary embryonic axis in Xenopus. Mediolateral intercalation begi ns in a group of cells called the vegetal alignment zone, a subpopulat ion of cells in Spemann's organizer, and spreads through much of the m arginal zone. To understand the functions of the vegetal alignment zon e during gastrulation and axis formation, we have inhibited its format ion by disrupting microtubules with nocodazole in early gastrula embry os. In such embryos, mediolateral intercalation, involution and conver gent extension of the marginal zone do not occur, Although cell motili ty continues, and the anterior notochordal and semitic mesoderm differ entiate in the pre-involution marginal zone, posterior notochordal and semitic mesoderm do not differentiate, In contrast, microtubule depol ymerization in midgastrula embryos, after the vegetal alignment zone h as formed, does not inhibit mediolateral cell intercalation, involutio n and convergent extension, or differentiation of posterior notochord and somites, We conclude that microtubules are required only for orien ting and polarizing at stage 10 1/2 the first cells that undergo medio lateral intercalation and form the vegetal alignment zone, and not for subsequent morphogenesis. These results demonstrate that microtubules are required to form the vegetal alignment zone, and that both microt ubules and the vegetal alignment zone play critical roles in the induc tive and morphogenetic activities of Spemann's organizer, In addition, our results suggest that Spemann's organizer contains multiple organi zers, which act in succession and change their location and function d uring gastrulation to generate the anterior/posterior axis in Xenopus.