48
Xenopus
FIGURE 4.3 Xenopus laevis half embryos have powerful self-organizing properties that are revealed by bisection. On the left, drawing
of blastula embryos being bisected in the sagittal or dorsal/ventral directions with an eyelash knife. (A) Uncut whole embryo. (B) Right
half embryo cut sagittally; the maternal pigment is darker on the left side of the tailbud tadpole (while it is uniform in the whole embryo).
(C) Left half from the same blastula; pigment asymmetry at tailbud is concentrated on the right side. (D) Dorsal half embryo at tailbud
showing almost perfect scaling of the half embryo along the antero-posterior axis to form a well-proportioned tadpole. (E) Ventral half
embryo; since it lacks the Spemann organizer, it develops into a belly-piece consisting of ventral tissues, such as blood and lateral plate,
without any dorsal axis. All embryos were from the same experimental batch. The diagrams indicate how displacement by 90° of the
Spemann organizer explains the tissue regeneration and pigment asymmetry observed in twinned embryos ( Moriyama and De Robertis,
2018 ). Numbers indicate the four segments of the 16-cell embryo that were lineage-traced. After sagittal bisection, the dorsal-most segment 1 becomes juxtaposed to the ventral-most segment 4, which has high BMP and Wnt expression potential. The organizer is not yet
formed at midblastula when embryos are bisected, but by early gastrula, the new organizer (indicated by the red dot) forms 90° away from
its original D-V location.
Source: Embryo images from Moriyama and De Robertis, 2018; reproduced with permission from the Proceedings of the National Academy of Sciences USA.
organizer (such fragments were called belly-pieces by center genes and their communication with dorsal signals
Spemann) ( Figure 4.3E – E ’’).
over long distances has been incompletely explored. The
regulatory mechanisms by which the D-V and A-P axis
are entwined remain largely unknown. We do not know
4.8. FUTURE AVENUES OF RESEARCH
whether other morphogen gradient-felds will be regulated
The embryos of the frog Xenopus laevis provide a marvel- by opposing poles of high and low signaling by growth
ous biological material. The advances derived from study- factor pathways. How a multitude of extracellular signals
ing the signaling components that control dorsal-ventral such as Wnt, FGF, and BMP signals are integrated at the
patterning and their remarkable regeneration properties level of hard-wired intracellular protein phosphorylations
after experimental manipulations have been profound. to generate simple cell differentiation decisions is only
Many new molecules have been discovered and the nature starting to emerge. More cell biological aspects, such as
of a morphogenetic gradient dissected. As we approach the relation between embryonic induction and membrane
the centennial of the Spemann-Mangold experiments in traffcking, are at their infancy. The questions change, but
2024, many unknowns remain. The nature of the ventral the frog remains.
Xenopus
FIGURE 4.3 Xenopus laevis half embryos have powerful self-organizing properties that are revealed by bisection. On the left, drawing
of blastula embryos being bisected in the sagittal or dorsal/ventral directions with an eyelash knife. (A) Uncut whole embryo. (B) Right
half embryo cut sagittally; the maternal pigment is darker on the left side of the tailbud tadpole (while it is uniform in the whole embryo).
(C) Left half from the same blastula; pigment asymmetry at tailbud is concentrated on the right side. (D) Dorsal half embryo at tailbud
showing almost perfect scaling of the half embryo along the antero-posterior axis to form a well-proportioned tadpole. (E) Ventral half
embryo; since it lacks the Spemann organizer, it develops into a belly-piece consisting of ventral tissues, such as blood and lateral plate,
without any dorsal axis. All embryos were from the same experimental batch. The diagrams indicate how displacement by 90° of the
Spemann organizer explains the tissue regeneration and pigment asymmetry observed in twinned embryos ( Moriyama and De Robertis,
2018 ). Numbers indicate the four segments of the 16-cell embryo that were lineage-traced. After sagittal bisection, the dorsal-most segment 1 becomes juxtaposed to the ventral-most segment 4, which has high BMP and Wnt expression potential. The organizer is not yet
formed at midblastula when embryos are bisected, but by early gastrula, the new organizer (indicated by the red dot) forms 90° away from
its original D-V location.
Source: Embryo images from Moriyama and De Robertis, 2018; reproduced with permission from the Proceedings of the National Academy of Sciences USA.
organizer (such fragments were called belly-pieces by center genes and their communication with dorsal signals
Spemann) ( Figure 4.3E – E ’’).
over long distances has been incompletely explored. The
regulatory mechanisms by which the D-V and A-P axis
are entwined remain largely unknown. We do not know
4.8. FUTURE AVENUES OF RESEARCH
whether other morphogen gradient-felds will be regulated
The embryos of the frog Xenopus laevis provide a marvel- by opposing poles of high and low signaling by growth
ous biological material. The advances derived from study- factor pathways. How a multitude of extracellular signals
ing the signaling components that control dorsal-ventral such as Wnt, FGF, and BMP signals are integrated at the
patterning and their remarkable regeneration properties level of hard-wired intracellular protein phosphorylations
after experimental manipulations have been profound. to generate simple cell differentiation decisions is only
Many new molecules have been discovered and the nature starting to emerge. More cell biological aspects, such as
of a morphogenetic gradient dissected. As we approach the relation between embryonic induction and membrane
the centennial of the Spemann-Mangold experiments in traffcking, are at their infancy. The questions change, but
2024, many unknowns remain. The nature of the ventral the frog remains.