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The Spemann organizer is largely responsible for organizing and patterning the anteroposterior axis during the development of amphibians. In this report, we examine the degree of anteroposterior pattern in the earliest gastrulaorganizer of Xenopus using a combination of embryological and molecular techniques. When we divide the earliest gastrulaorganizer, a region measuring 20 cells high by 25 cells wide, into stereotyped anterior (vegetal) and posterior (animal) halves, each half not only has a distinct fate and state of specification, but also induces a unique set of region-specific neural genes. When wrapped in animal capectoderm, the anterior half induces only anterior-specific genes (XAG-1 and otxA), while the posterior half induces anterior (otxA and reduced levels of XAG-1) and posterior (Hox B9) neural genes, revealing early localization of neural posteriorizing activity to posteriormesendoderm. This is the earliest demonstration of regionalized neural induction by the Xenopus organizer. Additionally, based on the expression of gsc, Xbra, and Xnot, we show that the organizer is patterned both at the early gastrula stage and prior to the appearance of bottle cells.
FIG. 1. Explanting the organizer and dividing it in half. The organizer was explanted from very early gastrulae (stage 10- through
stage 10); a sagittal section of a stage 10- early gastrula is shown. In a stage 10- embryo only the first few bottle cells have formed
from the epithelial layer of the organizer and little invagination has yet occurred. The explanted whole organizer was laid flat and
then cut in half with an eyebrow knife to separate prospective anterior (A) and prospective posterior (P) tissues. The dimensions of the organizer in units of cells are approximately as indicated.
FIG. 2. Secondary structures resulting from the implantation of whole Spemann organizer and its anterior and posterior halves into the ventralblastocoel at the early gastrula stage. The dorsal side of the primary axis is up. (A) The whole organizer induces an entire secondary body axis on the ventral side of the host embryo; (B) the anteriororganizer induces a secondary head, containing an eye and cement gland; (C) the posteriororganizer induces a second-ary trunk/tail; (D) a sector of ventralmarginal zone is noninductive; and (E) an unoperated tadpole at stage 39–40.
FIG. 3. Whole and half organizers were wrapped in late stage 9/early stage 10 animal cap ectoderm (wraps) and examined for expression
of otxA (blue) and Hox B9 (pink) by whole-mount in situ hybridization. (A) A normal tadpole at stage 25 expresses both genes; (B) isolated
animal caps never express otxA and occasionally express Hox B9 (not shown; see Table 1); (C) whole organizers induce both genes; (D)
anterior organizers induce otxA but no significant Hox B9; and (E) posterior organizers induce both genes.
FIG. 4. Wraps were prepared as usual and examined for expression of
the extremely anteriorXAG-1 gene. (A) A tadpole at stage 25 expresses XAG-1 in the cement gland (arrowhead); (B) isolated animal caps rarely express the gene autonomously; (C) whole organizers and (D) anterior organizers both strongly induce XAG-1; and (E) posterior organizers induce less XAG-1.
FIG. 5. Expression of gsc, Xbra, and Xnot in whole early gastrulae fixed at stage 10/ (A–C), in organizers explanted at stage 10- or stage
10 (D–I), and expression of gsc and Xbra in the late blastula (J). All explanted organizers are shown with posterior (animal) edge up and
anterior (vegetal) edge down. The shapes of organizers shown in side view in G–I vary due to slight variations in age; the organizer in G
was explanted at stage 100, and those in H and I at stage 10 (see Materials and Methods for details on how organizer shape varies with
time of explanation). (A) gsc is expressed in the vegetal half of the dorsal lip (arrowhead) and in deep endodermal cells at and below the
dorsal blastocoel floor; (B) Xbra is expressed throughout the marginal zone, but is excluded from cells above the dorsal lip (between
arrowhead and arrow); (C) Xnot is expressed throughout the animal hemisphere and is also excluded from cells above the dorsal lip
(between arrowhead and arrow); (D) gsc is expressed in the anterior (vegetal) half of the organizer only; (E) Xbra is expressed in the medial
region of the organizer; (F) Xnot is expressed within the posterior (animal) half of the organizer; (G) the gsc (purple, between arrowheads)
and Xnot (blue, between arrows) expressing domains share a common boundary in the center of the organizer; (H) Xbra (blue, between
arrowheads) partially overlaps gsc (pink, between arrows); (I) Xbra (purple, between arrowheads) partially overlaps Xnot (blue, between
arrows); and (J) in the late blastula, gsc (purple) and Xbra (blue) are expressed in separate yet overlapping compartments of the dorsal
marginal zone. Scale bars indicate relative magnifications.
