Chapter 2 From transplant to transcript

Abstract

This chapter discusses the developments that provide the understanding of flow of information from transplant to transcription. The first leap in molecular analysis towards “transcript” was finding a factor that specified mesoderm in animal caps. With the discovery of activin, the jump from transplant to transcript had been achieved. The remarkable progress from transplant to transcript has been most striking in the development of the vertebrate limb. The first important signaling region to be discovered in the chick limb was the apical ectodermal ridge. This thickened epithelium at the tip of the limb bud can be seen as a transparent rim. Work by researchers showed that when an apical ridge is grafted to the dorsal surface of a limb bud, a new outgrowth perpendicular to the normal limb is induced that can go on to form a limb tipped with digits. This result confirmed that the apical ridge has special properties that are necessary for laying down the proper sequence of structures along the long axis (proximo-distal axis) of the limb. The mechanisms that lead to patterning of the limb have been the subject of intense speculation and this problem is examined through the experiments' that involve vertebrate Shh gene and expression patterns of the first Hox genes.