Figure 1.

Proliferation and initial differentiation of interneuron precursors in the PWM. a–c, Distribution of dividing interneuron precursors in Pax-2-GFP transgenic mice at three different postnatal ages (P2, P6, P9). a, b, Graphs illustrate the frequency of GFP/BrdU double-labeled cells in PWM and cortical layers, 2 h (a) or 24 h (b) after BrdU administration (n = 2–4 cases/time point). c–e, The majority of dividing cells are located in the PWM (arrows in c). This distribution is confirmed by immunostaining for phospho-histone Ph3 (d, e). f, Laminar position of GFP/BrdU-double-labeled cells at increasing times after BrdU injection: 4 d after nucleotide analog administration, many cells are still in the PWM. g–n shows Pax-2 immunolabeling (g–j) or Pax-2/GFP expression (k–n) in interneuron precursors labeled by BrdU 2 h (g, h, k, l) or 24 h (i, j, m, n) before animals were killed. o, p, Double-labeled cells (arrows) consistently show a weaker Pax-2 expression at 2 h than at 24 h, as confirmed by the quantitative estimation of Pax-2 immunofluorescence intensity (o, Student's t test, p < 0.001). p shows the PWM of a P5 Gad67-GFP mouse: double immunostaining for GFP (green) and Pax-2 (red) reveals numerous cells positive for Pax-2 (arrowheads) but negative for GFP. q, r display double immunolabeling for GFP (green) and BrdU (red) in P5 Gad67-GFP mice. Double-labeled cells are not present 2 h after injection (q), whereas they are evident at 24 h (arrows in r). EGL, External granular layer; ML, molecular layer; GL, granular layer. Scale bars: (c, e, p–r), 20 μm; (g-n), 10 μm.