Granule neurons generated during development extend divergent axon collaterals to hippocampal area CA3

Most excitatory intrahippocampal pathways are characterized by significant, highly ordered projections into the long, or septotemporal, hippocampal axis. However, the mossy fiber system, the excitatory projection by which the dentate gyrus projects to hippocampal area CA3, is considered an exception, being organized to innervate exclusively transversely oriented cortical layers of the hippocampus. In the present study, the anatomy of the rat mossy fiber system was investigated using axonal tracing techniques, with an emphasis on determining its projection pattern into the long hippocampal axis. To this end, we used dual ipsilateral retrograde tracer injections to determine whether individual granule cells extend divergent axon collaterals to septotemporally distinct levels of hippocampal area CA3. We combined this technique with the fluorescent immunohistochemical detection of 5-bromo-2'-deoxyuridine (BrdU), a marker of granule cell precursors and their progeny, to address whether the divergence of mossy fiber collaterals within area CA3 might by related to ontogenic gradients in granule cell genesis. We observed single granule neurons that had retrogradely transported both tracers, indicating that they had axon collaterals passing through or terminating within the distinct levels of area CA3 into which tracer had been injected. By using BrdU labeling, we identified divergent granule neurons that were produced during embryonic and postnatal development. We observed no adult-generated granule neurons with significantly diverging mossy fiber collaterals. However, many fewer cells were labeled with BrdU in adult-exposed animals. Because of this smaller sample, we cannot rule out the possibility that small numbers of divergent adult-generated granule cells exist. We conclude that a proportion of the hippocampal mossy fiber projection extends septotemporally divergent axon collaterals to hippocampal area CA3.