Numerous cells divide away from the third ventricle in the embryonic mouse thalamus. (A-F) Frontal sections of embryonic day (E)10.5 (A,B), E12.5 (C,D) and E14.5 (E,F) of wild-type mouse forebrain. Midline is to the left. Panels (A,B) and (C,D) each shows the same sections of triple immunohistochemistry for PH3, Olig3 and Neurog2. Panel (E) is more dorsal to panel (F). Numerous PH3-positive cells exist not only at the surface of the third ventricle (A, arrow) but also away from the ventricle (A-D, arrowheads). At E14.5, when thalamic neurogenesis is almost over, a small number of PH3-positive cells are found in dorsal sections (E), but not in more ventral sections (F). Scale bar: 100 μm. PT, pretectum; C, thalamic progenitor domain pTH-C; R, thalamic progenitor domain pTH-R; ZLI, zona limitans intrathalamica. (G-I) Cell count of PH3-positive cells in the thalamus. (G) Proportion of PH3-positive cells located more than 40 μm away from ventricular surface (basal PH3+ cells) was calculated for E11.5, E12.5 and E14.5. The ratio of basal PH3+/total PH3+ cells was significantly higher at E12.5 than E11.5 or E14.5 (*P < 0.05 and **P < 0.01, respectively; n = 4 for E11.5, E12.5; n = 6 for E14.5). (H) At E12.5, a significantly higher ratio of basal PH3+/total PH3+ cells was found in pTH-C compared to pTH-R and ZLI domains (***P < 0.001; n = 4 for pTH-C; n = 6 for pTH-R and ZLI) (I) Distribution of PH3-positive cells in each of the 20-μm-wide medio-lateral bins, showing two distinct populations; one is located closest to the ventricular surface (bin 1) and the other is located more basally, with the peak at bin 6 (mean ± standard error of the mean; 5.0 ± 0.79).