The organization of the embronic and early postnatal murine hippocampus. II. Development of entorhinal, commissural, and septal connections studied with the lipophilic tracer DiI

Abstract

We have analyzed the early development of the main hippocampal afferents in the mouse. Following injections of the lipophilic tracer 1-1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) in the entorhinal cortex, entorhinal axons were observed for the first time in the hippocampus at E15, in the white matter. At E17, entorhinal fibers arborized within the stratum lacunosum-moleculare. At subsequent stages entorhinal axons formed dense networks that were restricted to their appropriate termination zone in the lacunosum-moleculare. The first axons invading the fascia dentata were noticed at E19, their density increasing at later stages. These axons were mainly present in the outer molecular layer. This onset of entorhinohippocampal projections was corroborated by retrograde labeling data after injections in the hippocampus. Commissural fibers first entered the contralateral hippocampus at E18, their number increasing at the following stages. Commissural axons arborized within the stratum oriens and radiatum in the hippocampus proper. In the fascia dentata, the earliest commissural fibers were seen at P2, terminating in the inner zone of the molecular layer and in the hilus. We conclude that developing entorhinal and commissural axons show a high degree of laminar specificity from the earliest stages of formation, which is compatible with the notion that distinct subsets of early maturing neurons populating the hippocampal plexiform layers may attract particular fiber systems. Hippocamposeptal fibers develop at E15, before the first septal fibers can be detected in the hippocampus. These early hippocamposeptal fibers originated from nonpyramidal neurons and terminated in the medial septal area, which is the main source of septal afferents to the hippocampus. In contrast, septohippocampal fibers were not seen in the hippocampus until E17. At perinatal stages, the hippocamposeptal connection reshapes, sending axons to the dorsolateral septal area as the innervation of the medial septum becomes less conspicuous. This sequence suggests that hippocampal neurons pioneer the formation of septohippocampal connections.

TrkB Signaling Is Required for Postnatal Survival of CNS Neurons and Protects Hippocampal and Motor Neurons from Axotomy-Induced Cell Death

“…Therefore, these neurons are axotomized during the preparation of organotypic hippocampal cultures. In contrast, most CA1 neurons are still migrating at this time, and few of their axons have entered the target fields (Bayer, 1980;Supèr and Soriano, 1994).…”

“…Organotypic cultures of hippocampal slices of neonatal trkB (Ϫ/Ϫ) mice display increased cell death in the CA3 region and the subiculum but not in the CA1 region or the dentate gyrus. CA3 and subiculum neurons project to the contralateral hippocampus and entorhinal cortex, respectively, and their axons reach their targets before birth (are among the first to be generated in the hippocampus) (Bayer, 1980;Supèr and Soriano, 1994). Therefore, these neurons are axotomized during the preparation of organotypic hippocampal cultures.…”

Basal forebrain cholinergic cell attachment and neurite outgrowth on organotypic slice cultures of hippocampal formation

“…Septal cholinergic projections to the dentate gyrus terminate principally in the inner molecular layer, although termination also occurs in the outer molecular layer and, to some extent, within the hilus (Amaral and Witter, 1995;Deller et al, 1999;Makuch et al, 2001;Milner et al, 1983;Super and Soriano, 1994). Patterns of BFCN distribution, as well as the pattern of AChEpositive neurite outgrowth, mimic the normal pattern of cholinergic a¡erents.…”

“…The projection from medial septum to the dentate gyrus displays one of the more clearly layered patterns of termination by cholinergic a¡erents (Amaral and Witter, 1995;Baratta et al, 1996a,b;Deller, 1998;Deller et al, 1999;Fo « rster et al, 1997;Makuch et al, 2001;Storm-Mathisen and Blackstadt, 1964;Super and Soriano, 1994;Zimmer et al, 1986), and it is this layered quality that makes this region an ideal model system for this study. Septal cholinergic projections to the dentate gyrus terminate principally in the inner molecular layer, although termination also occurs in the outer molecular layer and, to some extent, within the hilus (Amaral and Witter, 1995;Deller et al, 1999;Makuch et al, 2001;Milner et al, 1983;Super and Soriano, 1994).…”

“…The presence of AChE provides a convenient and sensitive endogenous marker for cholinergic axon growth (Baratta et al, 1996a,b;Levey et al, 1983). Third, the septal cholinergic projection to dentate gyrus develops largely postnatally in the rodent (Armstrong et al, 1987;Brady et al, 1989;Milner et al, 1983;Semba and Fibiger, 1988;Super and Soriano, 1994), making it easily accessible for experimental study.…”