Lack of barrel formation in Syngap^−/− mice. Nissl staining of flattened sections through layer 4 of P6.5 Syngap^+/+ (a) and Syngap^−/− (b) mice showing a complete absence of cellular aggregation in Syngap^−/− mice. Flattened sections through layer 4 of Syngap^+/+ (c), Syngap^+/− (d), and Syngap^−/− (e) mice were immunostained for 5-HT to reveal the distribution of TCAs. Clear segregation of primary visual (V1), somatosensory (S1), and auditory (A1) as well as secondary somatosensory (S2) cortical areas can be seen in all three genotypes indicating no general defect in TCA pathfinding in Syngap mutants. Within S1, the representation of different body regions (PMBSF and the anterior snout, lower lip, forepaw, and hindpaw representations) are also clearly defined (arrows). However, within the anterior snout, no TCA segregation is visible and, within PMBSF, TCAs can be seen segregating into rows, but patches corresponding to individual whiskers fail to form. Scale bar (in e): a, b, 1 mm; c–e, 800 μm. Quantification of neocortical area, area of S1, and PMBSF in Syngap^+/+, Syngap^+/−, and Syngap^−/− animals revealed a small but significant decrease in neocortical area and area of PMBSF in Syngap^−/− animals. The area of S1 was reduced in Syngap^−/− animals, although this decrease was not significant. No significant difference between Syngap^+/+ and Syngap^+/− was seen in any these measurements. In addition, there was no significant difference in the area of individual TCA patches of PMBSF barrels in Syngap^+/+ compared with Syngap^+/− mice (supplemental Fig. 1, available at www.jneurosci.org as supplemental material). Error bars represent SE.