Fig. 5.

Partition of the pioneer plate and establishment of the cortical plate. A–F, Stage 21, 7 GW. A–C, Sections from the more immature dorsal cortex at stage 21, stained for CR (A), GAD (B), and Reln (C).A, The pioneer plate, populated with intensely CR-ir neurons, is thin and contains many superficial horizontal cells, which resemble those of the monolayer at stage 20. B, GAD-ir neurons are less abundant than CR-ir cells and concentrated at the level of the superficial pioneer plate. C, Reln-ir neurons are located immediately below the pia.D–F are from the relatively more mature lateral cortex, stained for CR (D), GAD (E), and Reln (F).D, The lateral pioneer plate consists of several rows of densely aggregated, intensely CR-positive neurons that lack a radial orientation. E, GAD-ir neurons are less numerous than those expressing CR and more abundant in the external aspect of the pioneer plate. F, Reln is expressed specifically by cells in the subpial compartment. G–J, Stage 22, 8 GW, immunostained for CR. There is a gradual splitting of the pioneer plate from the most dorsal (G) to intermediate (H, I) and more lateral (J) levels. G, A pioneer plate stage similar to that shown in D, with the difference that an incipient radial cell orientation is now evident, as well as the presence of a fiber tract (small arrows point to axons). H–J, At progressively more lateral levels, the intensely CR-positive pioneer neurons become more and more separated into a superficial population (arrowheads) and a deep, radially oriented population that presumably represents the subplate (SP). The cortical plate (CP) consists of initially moderately CR-positive and later CR-negative neurons that become inserted within the pioneer plate. All microphotographs were taken at the same magnification. Scale bar (shown in C): 25 μm. Case 21, showing less perfect tissue preservation, could have undergone more retraction during histological procedures.