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 prim ordia in human embryos, the tegmental origin of the substantia nigra (SN) a nd ventral tegmental area (VTA) was located across several neuromeric domai ns: prosomeres 1-3, midbrain, and isthmus (Puelles and Verney, [1998] J. Co mp. Neurol. 394:283-308). The present study examines in detail the architec ture of the neural wall along this tegmental continuum in 6-7 week human em bryos, to better define the development of the SN and VTA. TH-immunoreactiv e (TH-IR) structures were mapped relative to longitudinal subdivisions (flo or plate,;basal plate, alar plate), as well as to radially superposed strat a of the neural wall (periventricular, intermediate, and superficial strata ). These morphologic entities were delineated at each relevant segmental le vel by using Nissl-stained sections and immunocytochemical mapping of calbi ndin, calretinin, and GABA in adjacent sagittal or frontal sections. A nume rous and varied neuronal population originates in the floor plate area, and some of its derivatives become related through lateral tangential migratio n with other neuronal populations born in distinct medial and lateral porti ons of the basal plate and in a transition zone at the border with the alar plate. Some structural differences characterize each segmental domain with in this common schema. The TH-IR neuroblasts arise predominantly within the ventricular zone of the floor plate and, more sparsely, within the adjacen t medial part of the basal plate. They first migrate radially from the vent ricular zone to the pia and then apparently move laterally and slightly ros tralward, crossing the superficial stratum of the basal plate. Several GABA -IR cell populations are present in this region. One of them, which might r epresent the anlage of the SN pars reticulata, is generated in the lateral part of the basal plate. (C) 2001 Wiley-Liss, Inc.