Figure 3. The trajectory of commissural fibers is determined by the position of the target granule cells. A, B, Cocultures of a wild-type (wt) hippocampal slice and a reeler mutant hippocampal slice. Commissural fibers projecting from the wild-type hippocampal slice to the reeler slice are visualized by tracing with biocytin; the injection site is marked by an asterisk. Note that commissural fibers do not form a sharp projection to the inner molecular layer of the reeler dentate gyrus (DG1) but intermingle with the malpositioned granule cells, suggesting that the trajectory of the commissural fibers is determined by the position of their target cells. As is characteristic for the reeler mutant, two pyramidal layers (p1, p2) are formed. Arrowhead points to the mossy fiber projection in the injected culture. p, Single pyramidal layer in the wild-type culture. The boxed area in A is shown at higher magnification in B. Scale bars: A, 100 μm; B, 50 μm. C-E, Triplet cocultures of two wild-type (wt) slices of hippocampus together with a reeler hippocampal slice. The biocytin injection site into one of the wild-type cultures is marked with an asterisk. Labeled commissural fibers invade the two other cultures, but only in the second wild-type culture do they show their characteristic compact termination in the inner molecular layer of the dentate gyrus (DG2). In the reeler hippocampal culture, the commissural fibers arising from the same cells of origin have lost their laminar specificity and terminate throughout the hilar region of the dentate gyrus (DG1). Arrowhead points to the mossy fiber projection in the injected culture. The boxed areas are shown at higher magnification in D and E. Scale bars: C, 100 μm; D, E, 50 μm. F, G, Coculture of reeler hippocampus and wild-type hippocampus. Biocytin was injected (asterisk) into the reeler dentate gyrus (DG1). Note that commissural fibers from the reeler dentate gyrus give rise to a compact projection to the inner molecular layer of the wild-type dentate gyrus (DG2). The boxed area in F is shown at higher magnification in G. Scale bars: F, 100 μm; G, 50 μm.