REGIONALIZATION OF THE FOREBRAIN FROM NEURAL PLATE TO NEURAL-TUBE

The vertebrate central nervous system is subdivided into distinct regi ons along the anteroposterior axis. It has been suggested that segment ation 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 hav e 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 inductio n of neural ectoderm. At the neural plate stage, distinct domains of g ene 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 o f morphological segment boundaries, is not thought to have occurred at the neural plate. When viewed in this context, segmentation may be co nsidered a relatively late event in the ontogenesis of the nervous sys tem. The early pattern of gene expression, the organization of the neu ral plate, and the underlying signals and tissue interactions may prov ide us with a better understanding of how the forebrain is regionalize d.