Spemann's organizer and self-regulation in amphibian embryos

Abstract

In 1924, Spemann and Mangold demonstrated the induction of Siamese twins in transplantation experiments with salamander eggs. Recent work in amphibian embryos has followed their lead and uncovered that cells in signalling centres that are located at the dorsal and ventral poles of the gastrula embryo communicate with each other through a network of secreted growth-factor antagonists, a protease that degrades them, a protease inhibitor and bone-morphogenic-protein signals.

Figure 1. Embryonic self-regulation

The entire early embryo constitutes a self-differentiating morphogenetic field, in which cells communicate with each other over great distances. This is demonstrated by experiments such as the one shown here, in which an African clawed frog (Xenopus laevis) embryo was cut into two halves at the blastula stage. If it is ensured that both halves contain part of the dorsal organizer region, two perfect identical twins are obtained (an intact sibling is shown at the top of the figure). In humans, identical twins are found in 3 out of 1,000 live births, and arise most frequently by the spontaneous separation of the inner cell mass of the mammalian blastula into two, followed by self-regulation. The ultimate example of self-regulation is provided by another mammal, the nine-banded armadillo, in which every blastocyst gives rise to four genetically identical siblings. Note that each twin is longer than just half the length of the intact sibling, which represents yet another effort to regulate towards the normal pattern. Both half-embryos shown here are derived from the same blastula.

Timeline

Birth, decline and rebirth of experimental embryology

Figure 2. Ubiquitous neural differentiation: epidermal differentiation can be restored by transplantation of either a dorsal or a ventral centre

Embryos were microinjected with four anti-Bmp (bone morphogenic protein) antisense morpholino oligos (against Admp (anti-dorsalizing morphogenic protein), Bmp2, Bmp4 and Bmp7) and stained for a neural (Sox2, top row) or an epidermal marker (cytokeratin, bottom row). a | An uninjected embryo that was stained for Sox2 mRNA and shows the normal central nervous system (CNS). b | After Bmp depletion, the entire embryo becomes covered uniformly by CNS tissue. c | Cytokeratin mRNA is abundantly expressed in skin. d | Cytokeratin mRNA disappears in Bmp-depleted embryos. e | When a ventral centre is grafted onto Bmp-depleted embryos (transplanted cells marked in red with the nuclear LacZ (nLacZ) lineage marker) the dorsal–ventral pattern is restored in part, even at a great distance from the graft. f | Surprisingly, the dorsal centre, or Spemann's organizer, also rescues the pattern: note that epidermis is induced, but at a considerable distance from the grafted tissue (the transplanted Spemann's organizer elongates because it gives rise to notochord). These experiments demonstrate: first, that Bmp inhibition causes neural induction and second, that the embryo has dorsal and ventral sources of Bmp signals. Reproduced, with permission, from REF. © (2005) Elsevier.

Figure 3. A network of interacting secreted proteins regulates dorsal–ventral cell communication

a | The dorsal centre secretes chordin and anti-dorsalizing morphogenic protein (Admp), whereas the ventral centre secretes bone morphogenic protein (Bmp)4 and Bmp7, bambi, crossveinless-2 (Cv2), sizzled and the xolloid-related (Xlr) metalloproteinase. Self-regulation of the dorsal–ventral pattern is mediated by dorsal and ventral proteins that are under reciprocal transcriptional regulation by Bmp signals,. Arrows in black indicate direct protein–protein interactions, arrows in blue indicate transcriptional-regulation steps that are mediated by the transcription factor Smad1, which is activated by Bmp signals. b | In the gastrula embryo, chordin mRNA is expressed in the dorsal blastopore lip and sizzled is expressed in the ventral blastopore lip. The circular blastopore closes during gastrulation, eventually giving rise to the anus.