Author

Hiroki Nishida

Published on SDB CoRe: Apr 10 2012

Early Embryogenesis: Blastula; Maternal Determinants
Embryonic Patterning: Axis Formation
Signaling: Secreted Signals
Mesoderm-derived: Notochord
Organism: Ascidian
Stage of Development: Embryo

Object Description

Ascidians (sea squirts) are invertebrate chordates. This figure shows how cell fates are specified in the vegetal hemisphere of a 32- to 64-cell stage ascidian embryo. Fibroblast growth factor (FGF, arrows) secreted from the endoderm blastomeres (yellow) at the 32-cell stage promotes asymmetric cell divisions in the marginal zone. The FGF signal is received on one side of the responding blastomeres resulting in formation of notochord (pink) in the anterior region, and mesenchyme (green) in the posterior region. The default fates (no FGF) are nerve cord (purple) in the anterior and muscle (red) in the posterior. Sister blastomeres are connected with a bar. The fact that a single signaling molecule (FGF) can promote the formation of two different fates (notochord and mesenchyme) is due to intrinsic factors within the signal-receiving cells. These competence factors are zygotic FoxA and Zic for notochord on the anterior side, and maternal macho-1 for mesenchyme on the posterior side. When macho-1-containing cells are induced by FGF, they assume a mesenchyme fate. In contrast, without maternal macho-1, zygotic expression of FoxA and Zic starts, and notochord is induced by FGF. WIREs Dev Biol 2011. DOI: 10.1002/wdev.22

References

Nishida, H. The maternal muscle determinant in the ascidian egg. WIREs Dev Biol, Published Online: Dec 27 2011.

Nishida, H. Patterning the marginal zone of early ascidian embryos: Localized maternal mRNA and inductive interactions. BioEssays, 2002, 24: 613-624.

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