Comments on the limits and internal structure of the mammalian midbrain

Abstract

The anatomic concept of the midbrain has changed considerably in recent times, due to advances in molecular developmental neurobiology which have shown the mechanisms that pattern the midbrain relative to diencephalon and hindbrain,as well as relative to its inner structure. The midbrain unitary developmental field that was discovered in this way is smallerthan the classic concept of the midbrain, which therefore necessarily included portions of the caudal diencephalon and therostral prepontine hindbrain. These developmental mechanisms are summarized, the correct boundaries defined, and thediencephalic and hindbrain structures that were wrongly ascribed to the midbrain are illustrated schematically and classifiedproperly as to their origins. Finally, the internal structure of the modern midbrain is treated systematically in terms ofdorsoventral and anteroposterior regionalization