Abstract
Marginal zone explants from Xenopus embryos can be used to expose cell behaviors and tissue movements that normally operate in dorsal tissues. Dorsal explants comprise the diverse set of progenitor cells found in dorsal tissues including mesendoderm, head mesoderm, prechordal mesoderm, endoderm with bottle cells, axial mesoderm of the prospective notochord, paraxial mesoderm of the somites, lateral plate mesoderm, neural ectoderm, and ectoderm. Unlike an organoid, the dorsal marginal zone (DMZ) explant is "organotypic" in that microsurgery does not disrupt native tissue organization beyond manipulations needed to dissect the tissue from the embryo. An organotypic early gastrula DMZ explant preserves boundaries and close tissue associations in the native marginal zone. Depending on the stage, patterning and cell identities can be maintained in explants and tissue isolates. Local cell movements and behaviors may also be preserved; however, the large-scale biomechanical impact of their collective movements may be altered from those in the native marginal zone. For instance, involution is typically inhibited in the DMZ explant, precluding the two-layer association of mesoderm and prospective neural ectoderm normally achieved during gastrulation. DMZ explants may be mounted and imaged in a variety of ways, exposing interesting cell behaviors or collective movements such as mediolateral cell intercalation in the axial and paraxial mesoderm, apical constriction of bottle cells, and directional migration of mesendoderm. The flattened DMZ explant can also be used to study emergence of new tissue-defining boundaries such as the notochord-somite boundary, the ectoderm-mesoderm boundary, and the mesendoderm-mesoderm boundary.