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 th e cortical plate formation (5, 6-7 and 7-8 gestational weeks (g.w.)). During this period, the telencephalic wall increased in width and lami nation pattern became complex while cellular morphology reflected prog ressive differentiation. At 5 g.w. (Carnegie stage 16) TW was 140 mum wide and had two zones, marginal (MZ) and ventricular (VZ). It was bui lt exclusively of bipolar ventricular cells and their upward directed processes. One week later, at 6 g.w. (stages 17 and 18) the telencepha lic wall displayed three zones (VZ, intermediate-IZ, MZ) and increased to 220 mum. 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 ori ented vertically in respect to the outer surface. However, horizontall y oriented cells were also noticed either at the VZ/IZ border or immed iately below the pia, where they probably represented Cajal-Retzius ne urons. 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 p art or a presubplate layer (IZ1) and a deep part or a future white mat ter (IZ2). Thus, at this stage, TW consisted of five zones (VZ, subven tricular, IZ with the presubplate, cortical plate-CP and MZ). The widt h of TW increased to 370-400 mum. 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 pre subplate displayed a variety of morphological forms. The presence of h orizontally oriented cells on the border of ventricular and subventric ular zone was noteworthy. Impregnated bundles of horizontal fibres wer e encountered in the IZ and MZ of the telencephalic wall at 6 g.w. The ir number increased in 7- to 8-g.w. embryos, especially in the lower ( future white matter) and upper (future subplate) intermediate zone. Ou r results on the cellular development of the cortical anlage, as revea led by Golgi method, not only support results obtained on different ex perimental animals but also add new relevant evidence about human deve lopment. Data on transition of simple bipolar ventricular cells to dif ferent neuronal shapes as constituents of emerging new laminae were no t available for human cortical anlage.