Hastings Nicholas B, Seth Malika I, Tanapat Patima, Rydel Tracy A, Gould Elizabeth
Department of Psychology, Princeton University, Princeton, New Jersey 08544, USA.
J Comp Neurol. 2002 Oct 28;452(4):324-33. doi: 10.1002/cne.10386.
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.
大多数海马体内的兴奋性通路的特征是，向长轴或颞隔海马轴发出显著且高度有序的投射。然而，苔藓纤维系统，即齿状回投射至海马CA3区的兴奋性投射，被认为是个例外，其组织方式是专门支配海马横向排列的皮质层。在本研究中，我们使用轴突追踪技术研究了大鼠苔藓纤维系统的解剖结构，重点是确定其在海马长轴上的投射模式。为此，我们使用双侧同侧逆行示踪剂注射来确定单个颗粒细胞是否将发散的轴突侧支延伸至海马CA3区在颞隔方向上不同的层面。我们将该技术与5-溴-2'-脱氧尿苷（BrdU）的荧光免疫组织化学检测相结合，BrdU是颗粒细胞前体及其后代细胞的标志物，以探讨CA3区内苔藓纤维侧支的发散是否可能与颗粒细胞发生过程中的个体发育梯度有关。我们观察到单个颗粒神经元逆向运输了两种示踪剂，这表明它们具有穿过或终止于注射示踪剂的CA3区不同层面内的轴突侧支。通过使用BrdU标记，我们鉴定出在胚胎期和出生后发育过程中产生的发散性颗粒神经元。我们未观察到具有明显发散性苔藓纤维侧支的成年期产生的颗粒神经元。然而，在成年期暴露的动物中，用BrdU标记的细胞要少得多。由于样本较小，我们不能排除存在少量发散性成年期产生的颗粒细胞的可能性。我们得出结论，海马苔藓纤维投射的一部分向海马CA3区发出在颞隔方向上发散的轴突侧支。
