The three-signal model for mesoderm induction. Although the demonstration that this third interaction is mediated by the interaction of opposing gradients of BMP ligands and BMP antagonists perhaps suggests that early induction and patterning of the mesoderm is best described by a four-signal model. A third, dorsalising signal (O) is derived from the dorsal marginal zone (Spemann’s organiser), which imparts the full range of mesodermal fates along the dorsoventral axis of the mesoderm during gastrula stages. In this model, there are two signals from the vegetal pole, a ventral signal (VV) and a dorsal signal (DV), which respectively induces extreme ventral and extreme dorsal mesoderm in the overlying marginal zone during blastula stages. A model well supported by embryological experiments is the three-signal model presented by Jonathan Slack ( Slack et al., 1987) and is shown in Figure 5. The mesoderm is derived solely from the animal cap cells and not the vegetal cells, indicating that the vegetal cells were providing a signal to change the fate of the responding animal cap cells to mesoderm. He showed that animal pole explants taken from a blastula-stage embryo (“animal caps”) cultured on their own give rise to surface ectoderm, however, when combined with vegetal cells will form mesoderm. Further studies describing the capacity of the vegetal hemisphere of urodele embryos to induce mesoderm in the adjacent marginal zone were carried out by Peter Nieuwkoop in the 1960s and 1970s. The central role for cell–cell signalling during embryonic development was thus established. In his most famous experiment, Spemann grafted the dorsal blastopore lip from a gastrula-staged amphibian embryo onto the ventral side of a differently pigmented host and found that the small graft itself gave rise only to a bit of notochord however, it induced the surrounding host tissue to form a second axis with a well-patterned neural tube and axial mesoderm. Embryonic induction was first described by Hans Spemann in the early 20th century.
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