Associational and commissural collaterals of neurons in the hippocampal formation (hilus fasciae dentatae and subfield CA3).

In rats, True Blue (or Granular Blue) was injected into the hippocampus of one hemisphere and Nuclear Yellow into a homotopic site of the hippocampus of the contralateral hemisphere. Following restricted injections into the septal part of area dentata fluorescent neurons were found in hilus fasciae dentate on both sides except for the most temporal 2 mm, while no fluorescent neurons were found in the Ammon's horn. Following restricted injections into the septal part of CA1, fluorescent CA3 pyramidal cells were found on both sides as far as 5.5--6.0 mm temporal to the injection, while no fluorescent neurons were seen in the hilus fasciae dentate. Hilus neurons simultaneously labeled with the True Blue and Nuclear Yellow were seen following injection of these substances into the area dentata of the two hemispheres, one tracer in each hemisphere, while double-labeled CA3 pyramidal cells were seen in cases with analogous injections in the septal part of CA1 of the two hemispheres. We conclude that the commissural and associational fibers to the area dentata and the Ammon's horn arise from neurons in the hilus and the CA3, respectively, and that at least some hilus and CA3 neurons have both an associational and a commissural branch.