Structure of longitudinal brain zones that provide the origin for the substantia nigra and ventral tegmental area in human embryos, as revealed by cytoarchitecture and tyrosine hydroxylase, calretinin, calbindin, and GABA immunoreactions.

In a previous work, mapping early tyrosine hydroxylase (TH) expressing primordia in human embryos, the tegmental origin of the substantia nigra (SN) and ventral tegmental area (VTA) was located across several neuromeric domains: prosomeres 1-3, midbrain, and isthmus (Puelles and Verney, [1998] J. Comp. Neurol. 394:283-308). The present study examines in detail the architecture of the neural wall along this tegmental continuum in 6-7 week human embryos, to better define the development of the SN and VTA. TH-immunoreactive (TH-IR) structures were mapped relative to longitudinal subdivisions (floor plate, basal plate, alar plate), as well as to radially superposed strata of the neural wall (periventricular, intermediate, and superficial strata). These morphologic entities were delineated at each relevant segmental level by using Nissl-stained sections and immunocytochemical mapping of calbindin, calretinin, and GABA in adjacent sagittal or frontal sections. A numerous and varied neuronal population originates in the floor plate area, and some of its derivatives become related through lateral tangential migration with other neuronal populations born in distinct medial and lateral portions of the basal plate and in a transition zone at the border with the alar plate. Some structural differences characterize each segmental domain within this common schema. The TH-IR neuroblasts arise predominantly within the ventricular zone of the floor plate and, more sparsely, within the adjacent medial part of the basal plate. They first migrate radially from the ventricular zone to the pia and then apparently move laterally and slightly rostralward, crossing the superficial stratum of the basal plate. Several GABA-IR cell populations are present in this region. One of them, which might represent the anlage of the SN pars reticulata, is generated in the lateral part of the basal plate. Copyright 2001 Wiley-Liss, Inc.