To identify laminar signatures at 21pcw, we correlated each gene with a binary vector (1 in tested layer versus 0 elsewhere), which identified ~2000 layer-enriched genes with R>0.5 in both brains (Suppl. Table 3). Each layer included genes with high laminar specificity (Fig. 2d), although the SZi profiles tended to overlap with the neighboring SZo and VZ. ISH validated the specificity of layer enriched genes (Fig. 2e). For example, the Cajal-Retzius cell marker CALB230 showed enrichment in MZ and SG as expected. The cortical progenitor markers TBR2 and PAX6 are enriched in germinal layers as in mouse, although PAX6 in mouse is restricted to VZ whereas it is also highly expressed in human SZ31. VZ-restricted GFAP expression likely marks radial glia (RG)32. Finally, expression of ZIC1, associated with Dandy-Walker congenital brain malformation33, was restricted to the pia mater overlying the cortex, therefore indicating that SG samples captured pial cells in addition to granule cells. However, while mouse Zic1 is expressed by virtually all Cajal-Retzius neurons34, our results indicate that this is not true in human as ZIC1 and CALB2 expression do not overlap in MZ (Figs. 1b and 2e).
