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A Practical Approach, Second Edition=Ronald D. Ho.pdf

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26 DEVELOPMENTAL REPRODUCTIVE TOXICOLOGY: A PRACTICAL APPROACH, SECOND EDITIONA.Locationof thelimb budsNeural tubeNotochordLimb budDorsal aortaCoelomB.ProximalAnteriorGremlinDistal BMP-2ShhFGFPosteriorProgress zone (PZ)of proliferating cellsApical ectodermal ridge(AER)Zone of polarizing activity(ZPA)C.DorsalWnt7aApical ectodermal ridge(AER)ProximalVentralDistalenenVentral expression of theengrailed gene blocksWnt7a gene expressionventrallyFigure 2.3Three-dimensional development of vertebrate limbs (tetrapod). This figure shows the three axesof limb development for four legged vertebrates: anteroposterior, proximodistal and dorsoventral.(A) Location of limb buds in relationship to an overall cross-sectional view of vertebrate development.(B) Cross-section of a limb bud showing the anteroposterior and dorsoventral axis. Thelocation of the zone of polarizing activity (ZPA) is shown, as is the apicalectodermal ridge (AER)and the progress zone (PZ) of proliferating cells. The signaling feedback between Sonic hedgehog(Shh), gremlin, BMP2, and FGF is illustrated. (C) Dorsoventral and proximodistal axis and therelationship of engrailed (en) gene and its inhibition of Wnt7a gene expression ventrally. (Adaptedfrom National Research Council, Scientific Frontiers in Developmental Toxicity Risk Assessment,National Academy Press, Washington, D.C., 2000, p. 354. With permission.)Table 2.2, researchers can determine, for example, if the temporal relationship, dose, and consistencyin changes in cell proliferation of the PZ correlate with the incidence of phocomelia.Likewise, researchers can determine the temporal relationship, dose, and consistency of specificlimb abnormalities by integrating gene expression changes induced with thalidomide. Because ofthe cross-species similarity in appendage or limb patterning, other approaches for mechanisticstudies could include looking at the consistency of these observations in Drosophila and chickembryos, as similar conserved gene domains exist. For example, similar domains exist for arthropodsand chordates for WG-HH-DPP and Wnt-Shh-BMP as do similar expressions of En, Ap, andLMX selector genes. 1 One caution that must be kept in mind when performing cross-species© 2006 by Taylor & Francis Group, LLC

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