3A. Standard or "Keller" explant of dorsal tissue from an early gastrula stage X. tropicalis embryo. Arrow denotes mesodermal/endodermal region and the arrowhead the presumptive neural region.
3B, 3C. After incubation for several hours (B) or for an additional period (C) one sees extensive convergence and extension movements in the mesodermal/endodermal region (arrows), as well as in the neural region (arrowheads) as in X. laevis.
3D. An example of such an explant from X. laevis labeled as in the previous example.
3E. Higher magnification of an "open-faced" explant of X. tropicalis used to study cell behaviors during dorsal tissue morphogenesis in X. laevis.
3F, G, H, I, J. X. tropicalis Is Amenable To Embryological Manipulation.
Prospective lens ectoderm transplants
The tolerance of amphibian tissues to transplantation from one embryo to another is useful in defining when and where signals are transmitted during inductive tissue interactions. For example, in the eye, lens is instructed to form by signals from adjacent presumptive retina. Transplantation assays in X. laevis have shown that ectoderm can only respond to these signals during a short period in development. We repeated these assays in the smaller X. tropicalis to establish that this species was experimentally accessible.
Lens-competent, gastrula stage ectoderm from a fluorescein-dextran-labeled donor embryo was transplanted to the presumptive lens-forming region of a neural plate stage host. As in X. laevis, labeled donor ectoderm went on to form a lens in roughly 50% (4 of 9) of the cases. The host embryo (top left) and site of transplanted ectoderm (viewed by fluorescence microscopy, top right) at the end of the experiment are shown. Sections through the head at this stage (bottom left) show the donor tissue labelling (bottom middle). The tissue identity of the transplanted lens was confirmed by staining with an antibody to X. laevis ?-crystallin, an abundant lens protein (bottom right). This experiment suggests that embryological techniques developed in X. laevis are readily adapted to X. tropicalis. Neither the slightly smaller size of the embryo nor the slightly higher temperatures required for development posed a significant impediment to the transplant procedure.
3F, 3G. Brightfield and fluorescence images, respectively, of X. tropicalis tadpole into which a piece of fluorescein dextran-labeled ectoderm had been tranplanted to the presumptive lens site at the neural plate stage.
3H. Nomarski image of a section through the head of tadpole in 3F. Arrow denotes position of an induced lens.
3I. Fluorescence image of section in H showing the transplanted tissue (fluorescein-labeled) at the left. Arrow denotes induced lens.
3J. Fluorescence image of section in H showing crystallin antibody staining of induced (arrow) and control lenses. In I and J fluorescence behind the right eye is a histological artifact.