Antennapedia Gene Expression (p. 534)
Activin gradient (pp. 543-546)
p.545, Fig. 21.20. The headline for the second column should read "Blastula dissection", rather than "Blastula (injection)".
Smith J.C. and Gurdon J.B. (2004) Many ways to make a gradient. BioEssays 26: 705-706
Tabata T. and Takei Y. (2004) Morphogens, their identification and regulation. Development 131: 703-712.
Teleman A.A., Strigini M. and Cohen S.M. (2001) Shaping morphogen gradients Cell 105: 559-562
Chen Y. and Schier A.F. (2001) The zebrafish Nodal signal Squint functions as a morphogen. Nature 411: 607-610
The authors investigated the morphogen properties of Cyclops and Squint, two transforming growth factor-ß signals required for mesoderm induction and patterning in zebrafish. After injecting mRNA for either protein into a single cell at the 128-256 cell stage, they used in situ hybrization to assay for the expression of known target genes including goosecoid+ (gsc+) and no tail+ (ntl+). They found that high doses of Squint induced the expression of both gsc+ and ntl+ at close range while ntl+ was also activated more distantly (up to 8 cells away from the Squint-producing cell). To test whether Squint acts through a relay mechanism, they used embryos mutant in the one-eyed pinhead (oep) gene, which do not respond to Squint. If squint mRNA was coinjected with oep mRNA into oep- mutants, then the injected cell expressed ntl+ but the adjacent cells did not. However, wild-type cells grafted close to the injected cell did express ntl+. Thus, the Squint signal can spread through unresponsive territory but does not spread by any relay mechanism independently of oep+ gene activity. Together, the results indicate that Squint acts as a morphogen.
Dubrulle J., McGrew M.J. and Pourquié O. (2001) FGF Signaling Controls Somite Boundary Position and Regulates Segmentation Clock Control of Spatiotemporal Hox Gene Activation. Cell 106: 219-232
The authors found that in chicken embryos fgf8 is transcribed only in tail cells, and that by growth and mRNA degradation a moving wave of FGF8 protein is produced. This wave seems to coordinate the segmentation process and spatiotemporal Hox gene activation.
Goetz JA, Suber LM, Zeng X, Robbins DJ. (2002) Sonic Hedgehog as a mediator of long-range signaling. Bioessays 24: 157-165
Gurdon JB, Bourillot PY. (2001) Morphogen gradient interpretation. Nature 413: 797-803.
Dubois L., Lecourtois M., Alexandre C.. Hirst E. and Vincent J.-P. (2001) Regulated endocytotic routing modulates wingless signaling in Drosophila embryos. Cell 105: 613-624
Most gradient models imply that the morphogen is synthesized at a localized source and inactivated as it moves away from the source. The rate of inactivation will then control the steepness of the gradient. This process was studied directly for the gradient of Wingless (Wg) protein in the ectoderm of the Drosophila embryo. The investigators used transgenic embryos producing a horeseraddish peroxidase-Wingless (HRP-Wg) fusion protein. The HRP moiety, which generates a dense stain in electron micrographs, persists in the cellular compartments involved in protein degradation, such as lysosomes, thus allowing the investigators to "see Wingless after it has been degraded". They found that Wg degradation is faster posterior than anterior of the strip of cells that synthesize Wg. Thus, the Wg gradient falls off steeply to the posterior while being more shallow to the anterior. The low concentration of Wg that is required to make denticles is therefore reached further away from the Wg source to the anterior than to the posterior (see Fig. 22.29 of textbook).
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