Fig. 3.

DiI diffusion is confined to the injected barrel.A, The layer 4 barrels are still visible in these bright-field micrographs (A1,A2) of a slice that has been microinjected with DiI, fixed in paraformaldehyde, and incubated at 35°C for 3 weeks to allow DiI diffusion. The DiI injection was confined to a diameter of 50 μm in the center of the middle barrel outlined in cyan (A2). DiI fluorescence images of the same slice (A3, A4) indicate that although the DiI has spread vertically toward the pia, the diffusion of DiI in the horizontal direction is confined to remain within the barrel boundary. Scale bar, 100 μm. B, The width at half-maximal intensity of the DiI fluorescence is well correlated to the width of the injected barrel (black line shows best linear fit by least squares method).C, The fluorescence intensity versus distance in the horizontal (within layer 4) or vertical (up toward the pia) was fitted by a sigmoidal function and the transition distance from 80% maximal to 20% maximal intensity is shown. At the barrel/septal boundary there is an abrupt decrease in DiI fluorescence, whereas the vertical spread decays slowly.