ABSTRACT
Many of the advances in understanding how the A/P axis is segmented occurred in the later part of the twentieth century as a result of the advances in techniques that allowed scientists to investigate the genetic and molecular aspects of development in greater detail. As methods of identifying genes and proteins continued to improve throughout the twentieth century, scientists were able to evaluate the molecular signals governing cellular development. In addition, methods for generating transgenic mice were introduced in the 1980s. This approach had an enormous impact on studies of A/P axis formation. As described in Chapter 1, transgenic mice are those that have an altered gene incorporated into a segment of DNA. Using transgenic mice, scientists are able to evaluate the effects of the altered gene on development, thereby providing insight into the normal function of the gene in neural development. There are limitations to this approach, some of which are especially important in developmental biology. In some cases, other related genes compensate for the missing gene. In other instances, the mutation is lethal to the embryo and the animal dies before the gene function can be determined. Despite these limitations, however, the use of transgenic mice has proved to be invaluable for understanding aspects of A/P segmentation along the vertebrate neural tube. These methods, and those developed for manipulating gene expression in chick embryos and Drosophila melanogaster, combined with classic methods of tissue grafting and cell culture, have elucidated many of the complex and
often shared mechanisms used to regulate segmentation of the A/P axis in multiple animal models.
