The epithelium of the dorsal marginal zone of Xenopus has organizer properties

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Abstract
We have investigated the properties of the epithelial layer of the dorsal marginal zone (DMZ) of the Xeno - pus laevis early gastrula and found that it has inductive properties similar to those of the entire Spemann orga- nizer. When grafts of the epithelial layer of the DMZ of early gastrulae labelled with fluorescein dextran were transplanted to the ventral sides of unlabelled host embryos, they induced secondary axes composed of notochord, somites and posterior neural tube. The orga- nizer epithelium rescued embryos ventralized by UV irradiation, inducing notochord, somites and posterior neural tube in these embryos, while over 90% of ven- tralized controls showed no such structures. Combina- tions of organizer epithelium and ventral marginal zone (VMZ) in explants of the early gastrula resulted in con- vergence, extension and differentiation of dorsal meso- dermal tissues, whereas similar recombinants of non- organizer epithelium and the VMZ did none of these things. In all cases, the axial structures forming in response to epithelial grafts were composed of labelled graft and unlabelled host cells, indicating an induction by the organizer epithelium of dorsal, axial morpho- genesis and tissue differentiation among mesodermal cells that otherwise showed non-axial development. Serial sectioning and scanning electron microscopy of control grafts shows that the epithelial organizer effect occurs in the absence of contaminating deep cells adher- ing to the epithelial grafts. However, labelled organizer epithelium grafted to the superficial cell layer con- tributed cells to deep mesodermal tissues, and organizer epithelium developed into mesodermal tissues when deliberately grafted into the deep region. This shows that these prospective endodermal epithelial cells are able to contribute to mesodermal, mesenchymal tissues when they move or are moved into the deep environ- ment. These results suggest that in normal development, the endodermal epithelium may influence some aspects of the cell motility underlying the mediolateral interca- lation (see Shih, J. and Keller, R. (1992) Development 116, 901-914), as well as the tissue differentiation of mesodermal cells. These results have implications for the analysis of mesoderm induction and for analysis of variations in the differentiation and morphogenetic function of the marginal zone in different species of amphibians.