Xenopus Embryo: Mesoderm Induction

Abstract

During early embryogenesis, the three germ layers, mesoderm, endoderm and ectoderm, must form in the correct positions for normal development to proceed. In the amphibian embryo, mesoderm is induced from cells of the marginal zone, at the equator of the spherical embryo, by signals emanating from the vegetal region. Later, during gastrulation, mesoderm is further patterned by signals from the dorsal organiser. Here, we review our current knowledge of mesoderm formation, highlighting the importance of the transforming growth factor‐beta (TGF‐β) family of extracellular signalling molecules in mesoderm induction and the role of maternal factors in activating these molecules. We also discuss mechanisms of limiting induction to the marginal zone by TGF‐β inhibitors in the animal hemisphere, the role of FGF signalling in maintaining the mesoderm and the role of BMP and BMP antagonists in patterning mesoderm during gastrulation.

Key Concepts

  • Mesoderm is induced to form from marginal zone cells of the Xenopus embryo in response to signals secreted from the vegetal hemisphere.
  • Mesoderm is induced by members of the Nodal/Activin sub‐family of the transforming growth factor‐beta (TGF‐β) family of secreted signalling factors.
  • Fibroblast growth factor (FGF) family members maintain mesoderm.
  • Mesoderm induction is limited to the marginal zone of the embryo by TGF‐β signalling inhibitors expressed in the animal region.
  • Maternal transcription factors, such as VegT and β‐catenin, activate expression of TGF‐β genes.
  • Mesoderm is further patterned during gastrulation through both activation and inhibition of BMP signalling.

Keywords: Xenopus; embryo; mesoderm; TGF‐β; Nodal

Figure 1. development. (a) early blastula (128 cells) is clearly divided into the darkly pigmented animal hemisphere (top) and the lightly pigmented vegetal hemisphere (bottom). (b) Cross section through late blastula‐stage embryo showing the large fluid‐filled blastocoel (bl) in the animal hemisphere. (c) Vegetal view of early gastrula‐stage embryo, showing the dorsal blastopore lip (dbl). (d) Three‐day‐old tadpole.
Figure 2. Mesoderm induction. (a) During blastula stages, the animal cap is specified as ectoderm, the marginal zone as mesoderm and the vegetal pole as endoderm (left). When the animal cap is grafted on to the vegetal pole (centre), the animal cap will also form mesoderm (right). The section on the right shows a 3‐day‐old animal cap that has differentiated a block of muscle (Mu). (b) Mesoderm is also induced in animal cap explants when they are incubated in media containing factors such as Activin (centre). In the absence of these factors, the animal cap forms a round ball of epidermis (−) but elongates (+) when incubated in sufficient Activin to induce dorsal mesoderm.
Figure 3. Three‐signal model for mesoderm formation. (a) Mesoderm is formed in the marginal zone of blastula‐stage embryos (left) as a result of two inductive signals. Most vegetal cells (VPs) release a signal (pink arrows) that induces ventral‐type mesoderm (M4), while a small group of cells called the Nieuwkoop centre (N) release a signal (red arrow) that induces the organiser (O). In addition, the animal pole expresses inhibitors of these signals (black barred lines) that limit the spread of mesoderm formation to the marginal zone. The organiser differentiates as dorsal mesoderm (M1) and releases signals (blue arrows; the third signal) that both dorsalise the adjacent marginal zone, specifying lateral mesodermal fates (M2 and M3), and induce neural tissue (NT) in the dorsal ectoderm. The remaining ectoderm differentiates as epidermis (Ep), whereas the vegetal pole differentiates as endoderm (En). (b–c) Whole‐mount hybridisation of early gastrulae, viewed from vegetal pole, showing expression patterns of (b) and (c).
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Further Reading

Kimelman D (2006) Mesoderm induction: from caps to chips. Nature Reviews Genetics 7: 360–72.

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Dale, Leslie, and Wardle, Fiona C(Jan 2015) Xenopus Embryo: Mesoderm Induction. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000730.pub3]