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 smaller than the classic concept of the midbrain, which therefore necessarily included portions of the caudal diencephalon and the rostral prepontine hindbrain. These developmental mechanisms are summarized, the correct boundaries defined, and the diencephalic and hindbrain structures that were wrongly ascribed to the midbrain are illustrated schematically and classified properly as to their origins. Finally, the internal structure of the modern midbrain is treated systematically in terms of dorsoventral and anteroposterior regionalization. Keywords: boundaries; derivatives; diencephalon; hindbrain; midbrain; neuromeres; patterning; regionalization