Cellular composition of the telencephalic wall in human embryos.

Cellular composition and lamination of the telencephalic wall (TW) was studied with Golgi impregnation in human embryos before and during the cortical plate formation (5, 6-7 and 7-8 gestational weeks (g.w.)). During this period, the telencephalic wall increased in width and lamination pattern became complex while cellular morphology reflected progressive differentiation. At 5 g.w. (Carnegie stage 16) TW was 140 microns wide and had two zones, marginal (MZ) and ventricular (VZ). It was built exclusively of bipolar ventricular cells and their upward directed processes. One week later, at 6 g.w. (stages 17 and 18) the telencephalic wall displayed three zones (VZ, intermediate-IZ, MZ) and increased to 220 microns. Sparse detached cells were found in the VZ while more of these cells were observed above VZ, in the incipient intermediate zone. These neurons were either simple or branched bipolar, and mainly oriented vertically in respect to the outer surface. However, horizontally oriented cells were also noticed either at the VZ/IZ border or immediately below the pia, where they probably represented Cajal-Retzius neurons. At 7-8 g.w. (stages 20-22), the cortical plate appeared between the IZ and the MZ at the ventrolateral level of the telencephalic wall. Intermediate zone could be further divided into a more superficial part or a presubplate layer (IZ1) and a deep part or a future white matter (IZ2). Thus, at this stage, TW consisted of five zones (VZ, subventricular, IZ with the presubplate, cortical plate-CP and MZ). The width of TW increased to 370-400 microns. Cortical plate cells were fusiform, with the apical dendrite branching into the MZ, and the axon directed in the opposite direction. Below it, differentiated neurons of the presubplate displayed a variety of morphological forms. The presence of horizontally oriented cells on the border of ventricular and subventricular zone was noteworthy. Impregnated bundles of horizontal fibres were encountered in the IZ and MZ of the telencephalic wall at 6 g.w. Their number increased in 7- to 8-g.w. embryos, especially in the lower (future white matter) and upper (future subplate) intermediate zone. Our results on the cellular development of the cortical anlage, as revealed by Golgi method, not only support results obtained on different experimental animals but also add new relevant evidence about human development. Data on transition of simple bipolar ventricular cells to different neuronal shapes as constituents of emerging new laminae were not available for human cortical anlage.