Notochord formation in amphibians: Two directions and two ways

Key characters uniting hemichordates and chordates: homologies or homoplasies?

Four chordate characters — dorsal hollow nerve cord, notochord, gill slits, and endostyle — are compared morphologically, molecularly, and functionally with similar structures in hemichordates to assess their putative homologies. The dorsal hollow nerve cord and enteropneust neurocord are probably homoplasies. The neurocord (= collar cord) may be an autapomorphy of Enteropneusta that innervates a unique pair of muscles, the perihemal coelomic muscles. Despite the apparent lack of organ-level homology, chordates and enteropneusts share a common pattern of neurulation that preserves a "contact innervation" between neuro- and myo-epithelia, which may be the primitive deuterostome pattern of neuromuscular innervation. The chordate notochord and hemichordate stomochord are probably homoplasies. Other potential notochord antecedents in hemichordates are examined, but no clear homolog is identified. The comparative morphology of notochords suggests that the "stack-of-coins" developmental stage, retained into adulthood only by cephalochordates, is the plesiomorphic notochord form. Hemichordate and chordate gill slits are probably homologs, but only at the level of simple ciliated circular or oval pores, lacking a skeleton, as occur in adults of Cephalodiscus spp., developmentally in some enteropneusts, and in many urochordates. Functional morphology, I125-binding experiments, and genetic data suggest that endostylar function may reside in the entire pharyngeal lining of Enteropneusta and is not restricted to a specialized midline structure as in chordates. A cladistic analysis of Deuterostomia, based partly on homologs discussed in this paper, indicates a sister-taxon relationship between Urochordata and Vertebrata, with Cephalochordata as the plesiomorphic clade.

Microtubule disruption reveals that Spemann's organizer is subdivided into two domains by the vegetal alignment zone

Mediolateral cell intercalation is proposed to drive morphogenesis of the primary embryonic axis in Xenopus. Mediolateral intercalation begins in a group of cells called the vegetal alignment zone, a subpopulation of cells in Spemann's organizer, and spreads through much of the marginal zone. To understand the functions of the vegetal alignment zone during gastrulation and axis formation, we have inhibited its formation by disrupting microtubules with nocodazole in early gastrula embryos. In such embryos, mediolateral intercalation, involution and convergent extension of the marginal zone do not occur. Although cell motility continues, and the anterior notochordal and somitic mesoderm differentiate in the pre-involution marginal zone, posterior notochordal and somitic mesoderm do not differentiate. In contrast, microtubule depolymerization in midgastrula embryos, after the vegetal alignment zone has formed, does not inhibit mediolateral cell intercalation, involution and convergent extension, or differentiation of posterior notochord and somites. We conclude that microtubules are required only for orienting and polarizing at stage 101/2 the first cells that undergo mediolateral 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 microtubules and the vegetal alignment zone play critical roles in the inductive and morphogenetic activities of Spemann's organizer. In addition, our results suggest that Spemann's organizer contains multiple organizers, which act in succession and change their location and function during gastrulation to generate the anterior/posterior axis in Xenopus.