Specification of midbrain territory.

The vertebrate neural plate is subdivided into four distinct territories comprising the presumptive forebrain, midbrain, hindbrain, and the spinal cord, shortly after gastrulation. Initially, this subdivision relies on a defined pattern of expression of distinct transcription and secreted factors within the newly formed neuroectoderm, even before morphological partitioning is evident. Subdivision of the neural plate into distinct territories is a complex process, which is also known as patterning or regionalisation, and involves both planar and vertical signals coming from within the neuroectoderm and from neighbouring non-neural tissues. During the course of embryogenesis, this gross subdivision of the neural plate is progressively refined by a variety of mechanisms, leading to the establishment of various subdomains that ultimately give rise to specific cell populations characteristic for the corresponding brain and spinal cord regions. Once again, a prominent feature of these later processes is the defined expression of specific genes within the developing neural tube. In the present review, we will concentrate on the genes active in the progressive refinement of the midbrain territory as a distinct subdivision of the brain. We will also give an outlook on genes that are active during early induction of the anterior neural plate and genetic mechanisms that control the generation of specific cell populations of the ventral midbrain, with special focus on the mesencephalic dopaminergic neurons.