Fig. 2. Bright-field photomicrographs of sections of cultured E19 cortex. a, Section of a slice of cortex that had been labeled with BrdU on E17 and cultured alone for 10 d; the section is reacted for BrdU and Nissl-stained. Dark dots at the pial edge of the slice are BrdU-labeled cells. Dotted lines mark the lower edge of the cortical plate. b, Nissl-stained section of slice that had been cultured alone for 3 d. Pyknotic cells were concentrated below the cortical plate (small dark dots). c, High-magnification view of the lower edge of the cortical plate in a 10 d Nissl-stained culture. The lower edge of the cortical plate is recognized by the transition from a region of high cell density to one of ∼50% lower density, which occurs over a depth of up to ∼50 μm (double-headed arrow); the density increases again in the remains of the ventricular zone (Gillies and Price, 1993b). A few pyknotic cells remain (dark dots); many appear more degenerate than after 3 d in culture. d, Section of a slice of cortex that had been labeled with BrdU on E13 and with tritiated thymidine on E17 and cultured with thalamus for 10 d. BrdU and tritiated thymidine-labeled cells were revealed, and the section was very lightly counterstained. BrdU-labeled cells were mainly in the subplate (dark dots); the dark dots in the cortical plate are tritiated thymidine-labeled cells. The groups of tritiated thymidine-labeled cells indicated by the open arrow in d are shown at higher magnification in e; note the typical appearance of the silver grains. The group of BrdU-labeled cells indicated by the solid arrow in d is shown at higher magnification in f. g, Section of a slice of cortex, labeled with BrdU on E13, that had been cultured with another slice of cortex for 10 d. Very few BrdU-labeled cells were present. The region between the dotted lines is where the two slices have fused.CP, Cortical plate; CTX, cortex; IZ, intermediate zone; SP, subplate; T, thalamus. Scale bars: a, b, d, g, 50 μm; c, e, f, 15 μm.