Fig. 3.

Cell populations in preplate, pioneer plate, and cortical plate. A–D, Stage 20, 7 GW.A, A loosely organized layer of mostly horizontal calretinin (CR)-ir neurons, referred to as the monolayer (ML, between the two parallel lines), appears in the preplate, superficial to the intermediate zone (IZ). Scale bar, 40 μm. B, Large neurons in the monolayer are strongly CR-ir and display a differentiated dendritic branching pattern. Arrowheadpoints to a dendritic ramification. Scale bar, 35 μm.A and B are from the dorsolateral cortical wall. C, In the more mature ventrolateral cortex, other CR-ir neurons begin to assemble at the inner aspect of the monolayer. Scale bar, 40 μm. D, An adjacent section, immunostained for Reln, shows that the neurons in the monolayer (between the parallel lines) do not express Reln, but that Reln-ir neurons are confined to an immediate subpial compartment (open arrowheads mark the position of the pial surface). Scale bar, 40 μm. E–H, Stage 22, 8 GW. E–G, Adjacent sections of the dorsal neocortex, a level where the pioneer plate (PP) is just segregating, immunostained for GAD (E), Reln (F), and CR (G). E, Horizontal GAD-ir neurons (solid arrowheads) lie immediately above the PP, in the deep tiers of the marginal zone (MZ), whereas the Reln-ir cell layer (F) has a subpial position.G, Neurons in the PP are strongly CR-positive.Open arrows indicate a strongly CR-ir plexus just above the PP. H, CR-stained section in the ventrolateral neocortex, where the cortical plate (CP) has split the PP into a superficial (solid arrowheads) and a deep contingent. Deep pioneer neurons, presumably representing the subplate (SP), assume a pyramidal shape but display a more differentiated dendritic branching pattern than CP neurons. CR is now strongly expressed by cells in the subpial compartment. Scale bars (E–H): 50 μm.