Genetic control of caudal development.

Several lines of evidence suggest that caudal development involves a distinct programme. This is illustrated by the fact that a specific pattern of malformations affects the caudal end of the human embryo. In addition, neurulation, the process leading to the formation of the neural tube, proceeds through different morphogenetic movements caudally. In mammals, as in birds, the caudal neural tube arises from cavitation and not from folding of the neural plate as in more rostral levels. However, recent fate mapping studies have suggested that the two modes of neurulation represent a continuous programme, possibly involving similar cellular or molecular mechanisms. Finally, analyses of mutant mice have shown that T-box transcription factors and components of the Wnt signalling pathway control cellular migration and the promotion of mesoderm formation in the caudal embryo. In humans, mutation in the HLXB9 transcription factor causes an autosomal dominant form of sacral agenesis. Thus, the combination of classical embryological and molecular genetics approaches has provided critical reference points for the delineation of the developmental programme of the caudal embryo.