Regionalization of the forebrain from neural plate to neural tube.

The vertebrate central nervous system is subdivided into distinct regions along the anteroposterior axis. It has been suggested that segmentation may play an important role in the regionalization and patterning of the neural tube, in particular in the development of the hindbrain. Recent evidence suggests that this may be true also in the forebrain. Several criteria that have been used to define segments and that have led to different views on how the anterior neural tube is subdivided will be discussed. However, regionalization of the neural ectoderm is a gradual and continuous process that starts as early as the induction of neural ectoderm. At the neural plate stage, distinct domains of gene expression have been established, indicating that a certain amount of regionalization has already taken place. Segmentation, as defined by lineage restriction between proposed segments or by the formation of morphological segment boundaries, is not thought to have occurred at the neural plate. When viewed in this context, segmentation may be considered a relatively late event in the ontogenesis of the nervous system. The early pattern of gene expression, the organization of the neural plate, and the underlying signals and tissue interactions may provide us with a better understanding of how the forebrain is regionalized.