Postnatal development of CA3 pyramidal neurons and their afferents in the Ammon's horn of rhesus monkeys

Previous studies described the postnatal development of CA3 pyramidal neurons and their afferents in the rat. However, the postnatal development of the primate hippocampus was not previously studied. Thus, pyramidal neurons of the CA3 area of the monkey hippocampus were analyzed postnatally in the present study. At birth, a few thorny excrescences, the complex spines postsynaptic to mossy fibers, were found on the proximal segments of both apical and basal dendrites, whereas distal dendrites displayed pedunculate spines. Thorny excrescences increased in number until the third month. A continuous increase in the number of spines per unit length along the distal dendrites was observed during the first 12 months. The ultrastructural features of somata and dendrites of pyramidal cells in newborn monkeys were similar to those of adults. The analysis of the afferents to the CA3 pyramidal neurons was limited to the development of mossy fibers, the axons of granule cells, and myelinated axons in the alveus, stratum oriens, and stratum lacunosum-moleculare. At birth, most mossy fiber terminals were densely packed with synaptic vesicles and formed mainly axospinous synapses with CA3 pyramidal cells. By 1 month of age, the number of mitochondria and embedded spines increased to mature amounts. In the first postnatal month, degenerating axons and axon terminals were frequently observed in the mossy fiber bundles in stratum lucidum. The proportion of myelinated axons increased simultaneously in all three examined layers. At birth most axons were unmyelinated, whereas at 7 months of age the proportion of myelinated axons was similar to that found in adults. The present study indicates that most pyramidal neurons of the CA3 region in monkeys are in an advanced stage of development at the time of birth. Thus, mossy fibers from granule cells in the dentate gyrus have established mature-looking synapses, and the thorny excrescences of pyramidal cells that are postsynaptic to mossy fibers are also adult-like. Nevertheless, several of the adult features, such as the spine density of distal dendrites of pyramidal neurons and the myelination of afferent axons, develop during an extended period of time in the first year. The significance of this early anatomical maturation in a brain region involved in memory function is consistent with recent behavioral data that show a rapid postnatal maturation of limbic-dependent recognition memory in rhesus monkeys. © 1995 Wiley-Liss, Inc.     

Fetal Hippocampal Cells Grafted to Kainate-Lesioned CA3 Region of Adult Hippocampus Suppress Aberrant Supragranular Sprouting of Host Mossy Fibers

Selective lesion of the rat hippocampus using an intracerebroventricular administration of kainic acid (KA) represents an animal model for studying both lesion recovery and temporal lobe epilepsy. This KA lesion leads initially to loss of CA3 hippocampal neurons, the postsynaptic target of mossy fibers, and later results in aberrant mossy fiber sprouting into the dentate supragranular layer (DSGL). Because of the close association of this aberrant mossy fiber sprouting with an increase in the seizure susceptibility of the dentate gyrus, delayed therapeutic strategies capable of suppressing the sprouting of mossy fibers into the DSGL are of significant importance. We hypothesize that neural grafting can restore the disrupted hippocampal mossy fiber circuitry in this model through the establishment of appropriate mossy fiber projections onto grafted pyramidal neurons and that these appropriate projections will lead to reduced inappropriate sprouting into the DSGL. Large grafts of Embryonic Day 19 hippocampal cells were transplanted into adult hippocampus at 4 days post-KA lesion. Aberrant mossy fiber sprouting was quantified after 3–4 months survival using three different measures of Timm's staining density. Grafts located near the degenerated CA3 cell layer showed dense ingrowth of host mossy fibers compared to grafts elsewhere in the hippocampus. Aberrant mossy fiber sprouting throughout the dentate gyrus was dramatically and specifically reduced in animals with grafts near the degenerated CA3 cell layer compared to “lesion only” animals and those with ectopic grafts away from the CA3 region. These results reveal the capability of appropriately placed fetal hippocampal grafts to restore disrupted hippocampal mossy fiber circuitry by attracting sufficient host mossy fibers to suppress the development of aberrant circuitry in hippocampus. Thus, providing an appropriate postsynaptic target at early postlesion periods significantly facilitates lesion recovery. The graft-induced long-term suppression of aberrant sprouting shown here may provide a new avenue for amelioration of hyperexcitability that occurs following hippocampal lesions.     

Comparative localization of enkephalin and cholecystokinin immunoreactivities and heavy metals in the hippocampus

The present study is concerned with the cellular origins and identities of the hippocampal enkephalin and CCK-immunoreactive fibers and terminals. In the hippocampus of the rat, the guinea pig and the European hedgehog a system of enkephalin immunoreactive nerves emerges in the hilus of area dentata and can be followed to the apical dendrites of the hippocampal regio inferior pyramidal cells. This pattern of immunoreactive nerves corresponds to the hippocampal mossy fiber system as visualized by the Timm staining. Cholecystokinin immunoreactive nerve fibers and terminals reveal the same distribution in the guinea pig and the European hedgehog whereas in the rat the mossy fiber zone contains little or no cholecystokinin immunoreactivity. In the guinea pig degeneration of the mossy fibers after stereotactic lesions of the mossy fibers causes a complete loss of both enkephalin and cholecystokinin immunoreactivity in the mossy fiber zone. Only a few enkephalin immunoreactive cell bodies were scattered throughout the granular cell layer of area dentata, but inhibition of the axoplasmic transport by colchicine dramatically increased the number of enkephalin immunoreactive granule cell bodies. Enkephalin immunoreactive cell bodies were also detected in the hilus, throughout the pyramidal cell layer as well as in the stratum radiatum and stratum moleculare. Cholecystokinin immunoreactive cell bodies were seen in the hilus of the area dentata and in the stratum oriens, stratum pyramidale and stratum radiatum and the cell-rich layer of subiculum. No cholecystokinin immunoreactive cell bodies were observed in the granular cell layer of area dentata. Even after colchicine treatment the granule cells were devoid of cholecystokinin immunoreactivity. In the rat a system of nerves displaying enkephalin immunoreactivity was also observed in the superficial one-third of the molecular layer of area dentata, a zone which corresponds to the termination of the lateral perforant path. Another observation was that in the rat, but not in the guinea pig and the hedgehog, the terminal zone of both the medial perforant path and the zone of commissural and associational fibers of area dentata contained cholecystokinin immunoreactive molecules. In summary, our data show: (1) that the hippocampal mossy fibers contain enkephalin immunoreactive molecules; (2) that the cholecystokinin immunoreactivity in the mossy fiber zone is most likely also localized in the mossy fibers per se, although the granule cells seem devoid of cholecystokinin immunoreactivity; (3) zinc, here visualized as a Timm-positive substance, is also localized in the mossy fiber terminals; further, (4) other intrinsic cell bodies than the granule cells may contribute to both the enkephalin and cholecystokinin immunoreactive terminals within the hippocampus; (5) in the rat the lateral perforant path may be enkephalinergic; and (6) both the terminal zone of the medial perforant path and the associational and commissural fibers of the rat contain cholecystokinin immunoreactivity.     

Opiate receptor subtype binding in gerbil hippocampus is altered by forebrain ischemia

A role for endogenous opioids in trauma-induced brain injury has been supported by pharmacological studies. The present series of experiments were initiated to extend these observations by measuring opiate receptor subtype binding in gerbil hippocampus following 7 days recovery from a 10 min ischemic insult. Quantitative in vitro autoradiography was utilized to measure mu [( 3H]DAGO), kappa [( 3H]bremazocine + 10 microM morphiceptin + 100 nM DSLET), delta [( 3H]DSLET + 10 microM morphiceptin) and lambda [( 3H]naloxone + 300 nM diprenorphine) binding. While ischemic tissue samples at the level of the dorsal hippocampus showed complete loss of CA1 pyramidal cells, we observed no significant alterations in mu or delta binding suggesting a non-pyramidal cell localization of these receptors. Kappa binding decreased significantly to 88% of control in the CA1 and CA3 regions while lambda binding in the stratum lucidum (CA3) increased to 165% of control. Our results show that opiate receptor subtypes are differentially affected by an ischemic insult.     

Specific hippocampal lesions indicate the presence of sulfonylurea binding sites associated to ATP-sensitive K channels both post-synaptically and on mossy fibers

The density of glibenclamide (GB) binding sites decreases considerably in the dentate gyrus (>80%), the CA1 field (75%) and the stratum lucidum of CA3 field (70%) after intradentate colchicine injections in rat hippocampus. The density of GB receptors is unchanged in kainic acid damaged hippocampus. These data show that GB binding sites associated to adenosine triphosphate-sensitive K⁺ channels are mainly located in granular cells in both pre- and post-synaptic positions. They are present in mossy fibers.     

The comparative immunocytochemical distribution of 28 kDa cholecalcin (CaBP) in the hippocampus of rat, guinea pig and hedgehog

The distribution of 28 kDa cholecalcin (calcium-binding protein, CaBP) in the hippocampal formation of the rat, guinea pig and European hedgehog was examined by immunocytochemistry. The extension of the mossy fibers (the axons of the granule cells of the dentate gyrus) was also studied using the Timm's sulfide-silver method. Cholecalcin was present in all mossy fibers. In the rat, only those pyramidal cells not reached by the labeled mossy fibers displayed cholecalcin immunoreactivity. Immunocytochemical staining of the hedgehog hippocampus showed that contacts between cholecalcin-containing mossy fibers and cholecalcin-containing pyramidal cells are possible. Consequently, the protein is probably not involved in the control of mossy fiber extension. Strikingly, no guinea pig pyramidal cells showed cholecalcin immunoreactivity. The possible involvement of cholecalcin in the differential excitability of pyramidal cells in the CA3 and CA1 areas of the hippocampus could therefore be tested in a comparative study of rat, guinea pig and hedgehog.     

海人酸大鼠癫痫模型的建立

目的 应用海人酸注射建立海人酸大鼠癫痫模型,评价其生物学特性.方法 通过立体定位手术,大鼠海马组织微量注射海人酸,术后观察大鼠行为学表现、电生理改变及海马超微形态结构变化.结果 海人酸注射后实验大鼠出现典型的颞叶癫痫发作过程,表现为湿狗样抖动、前肢抽搐、跌倒以及全身强直-阵挛性发作等,皮层脑电图出现多种形式的痫性放电,单神经元放电细胞外记录显示癫痫模型大鼠放电杂乱,形态不规整.电镜显示神经细胞固缩,星形细胞突起肿胀,神经突触水肿,可见兴奋性递质小泡,线粒体水肿崩解,嵴排列紊乱,血管内皮细胞水肿,血脑屏障破坏.结论 大鼠立体定向海人酸药物注射是一种较理想的癫痫模型.     

Changes in mint1, a novel synaptic protein, after transient global ischemia in mouse hippocampus.

Mints (munc18-interacting proteins) are novel multimodular adapter proteins in membrane transport and organization. Mint1, a neuronal isoform, is involved in synaptic vesicle exocytosis. Its potential effects on development of ischemic damage to neurons have not yet been evaluated. The authors examined changes in mint1 and other synaptic proteins by immunohistochemistry after transient global ischemia in mouse hippocampus. In sham-ischemic mice, immunoreactivity for mint1 was rich in fibers projecting from the entorhinal cortex to the hippocampus and in the mossy fibers linking the granule cells of the dentate gyrus to CA3 pyramidal neurons. Munc18-1, a binding partner of mint1, was distributed uniformly throughout the hippocampus, and synaptophysin 2, a synaptic vesicle protein, was localized mainly in mossy fibers. After transient global ischemia, mint1 immunoreactivity in mossy fibers was dramatically decreased at 1 day of reperfusion but actually showed enhancement at 3 days. However, munc18-1 and synaptophysin 2 were substantially expressed in the same region throughout the reperfusion period. These findings suggest that mint1 participates in neuronal transmission along the excitatory pathway linking the entorhinal cortex to CA3 in the hippocampus. Because mint1 was transiently decreased in the mossy fiber projection after ischemia, functional impairment of neuronal transmission in the projection from the dentate gyrus to CA3 pyramidal neurons might be involved in delayed neuronal death.     

Differential expression of S100beta and glial fibrillary acidic protein in the hippocampus after kainic acid-induced lesions and mossy fiber sprouting in adult rat

The expression of S100beta and glial fibrillary acidic protein (GFAP) was analyzed following bilateral injection of kainic acid (KA), a glutamate derivative, into the CA3 region of the adult rat hippocampus. This treatment produces a progressive degeneration of the pyramidal neurons of the hippocampus while sparing the granule cells of the dentate gyrus which undergo sprouting of their axons in the supragranular layer. Messenger RNA and protein levels were measured, by Northern blot and ELISA, in the hippocampus of lesioned and sham-operated rats 1, 7, and 30 days after KA injection. A significant increase of GFAP and its mRNA was demonstrated at each time point, whereas S100beta mRNA levels were significantly enhanced only 30 days after the KA injection and the levels of S100beta protein remained unchanged at all time points. However, when analyzed by immunohistochemistry the S100beta showed clear changes in its expression and distribution depending on the region considered. One month after KA injection, S100beta immunoreactivity was considerably reduced in the stratum radiatum of CA3 region, but there was increased S100beta immunoreactivity in the stratum moleculare. In particular, a notable band of S100beta positive, hypertrophic astrocytes appeared in the supragranular layer of the dentate gyrus where the sprouting of mossy fiber collaterals was detected by Timm's staining. These data show for the first time that an increase in S100beta expression in subpopulations of reactive astrocytes may be involved in the structural reorganization of the hippocampus following KA-induced neurodegeneration.     

Forebrain ischemia in the gerbil increases λ opiate binding in hippocampal mossy fibers

Transient forebrain ischemia was produced in gerbils by short-term occlusion of the common carotid arteries under halothane anesthesia. Histological analysis of brains 7 days post-ischemia demonstrated characteristic destruction of CA1 pyramidal cells. λ Opiate binding (measured with [³H]naloxone in the presence of 300 nM diprenorphine) at 7 days post-ischemia was significantly increased in the stratum lucidum of the hippocampus (the mossy fiber layer), but not in any other region measured, including other hippocampal regions, cortex, amygdala, caudate putamen, thalamus, and hypothalamus. The increase in mossy fiber λ binding was slow to develop (no increase detected up to 48 h post-ischemia), and long-lasting (binding remained elevated at 32 days post-ischemia). While MK-801 significantly inhibited CA1 pyramidal cell destruction when administered 20 min prior to ischemia, the increase in mossy fiber λ binding was still evident. None of seven different opioid agonists and antagonists examined had an effect on either the pyramidal cell damage or increased mossy fiber λ binding seen 7 days after ischemia.     

Ultrastructural alterations in hippocampal mossy fiber synapses in schizophrenia: a postmortem morphometric study

Synapses formed between mossy fibers, the axons of hippocampal dentate granular cells, and the dendrites of CA3 pyramidal neurons are important links within the trisynaptic circuitry. Abnormalities in this circuitry are associated with the failure of schizophrenics to integrate affective experience with higher cognitive function, and with disturbances in memory and spatial learning processes. The abnormalities include reduced size and altered dendritic arborization of CA3 pyramidal neurons. In addition, decreased expression and binding activity of glutamate receptors have been reported, predominantly in the CA3 region of the hippocampus. These findings suggest that there are disturbed neuronal processes and connections in the hippocampus of schizophrenics. An electron microscope morphometric study of synaptic contacts between mossy fiber axon terminals (MFT) and branched dendritic spines of pyramidal neurons in stratum lucidum of the CA3 region of the hippocampus was performed in 10 normal controls and 9 age-matched chronic schizophrenics (postmortem delay 3-9 h). Schizophrenic cases with predominantly positive symptoms had a significantly reduced volume fraction of spines (-35%, P < 0.05), total number of invaginated spines (-47%, P < 0.01), and number of spines forming synapses (-32%, P < 0.05) per MFT compared with the control group. No effects of postmortem delay, age, duration of disease, or neuroleptic exposure were found. These data may reflect decreased efficacy of mossy fiber synapses in the CA3 hippocampal region in schizophrenics with predominantly positive symptoms. These data are in line with the neurodevelopmental hypothesis of schizophrenia.     

Excitatory amino acid binding sites in the rat hippocampus after transient forebrain ischemia

The influence of transient forebrain ischemia on the temporal alteration of glutamate receptors in the hippocampal formation was analyzed by means of in vitro quantitative receptor autoradiography. We compared the binding of N-methyl-D-aspartate (NMDA) receptors using [3H]3-[+/-)2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP), noncompetitive NMDA antagonist binding sites using [3H]N-(1-(2-thienyl)-cyclohexyl)-3,4-piperidine (TCP), and kainate (KA) receptors. In the CA1 subfield of the hippocampus, the number of NMDA receptors and noncompetitive NMDA antagonist binding sites remained constant during the early stage of recirculation when the CA1 pyramidal cells remained histologically intact. A significant reduction of these receptor densities was observed 7 days following ischemia, when NMDA receptors and noncompetitive NMDA antagonist binding sites lost 64 and 29% of their binding sites in the stratum radiatum of the CA1, respectively. The KA receptor density in the CA1 subfield decreased by 44% 7 days after ischemia. Marked loss of the above-mentioned receptors in the CA1 after selective depletion of the CA1 pyramidal cells indicated that NMDA receptors, noncompetitive NMDA antagonist binding sites, and KA receptors in the CA1 are predominantly localized on the CA1 pyramidal cells. NMDA receptor density in the CA3 gradually decreased during the recirculation period. The stratum moleculare of the dentate gyrus, whose structure was histologically intact after ischemic insult, also showed a reduction of NMDA receptors 7 days following ischemia. [3H]KA receptor density in the stratum lucidum of the CA3 and in the hilus also decreased during recirculation. These     

Development of the mossy fibers of the dentate gyrus: I. A light and electron microscopic study of the mossy fibers and their expansions

The postnatal development of the axons of the dentate granule cells—the so-called mossy fibers—was studied at the light microscopic level in Timm and Golgi preparations and also by transmission electron microscopy. In the Timm-stained material, there was a distinctive coloration in the hilus and incipient stratum lucidum, indicating the presence of mossy fibers, on the first postnatal day. Over the next two weeks, the stained areas became more extensive, the size and density of the stained particles increased, and the particles became more intensely stained. These signs of progressive development of the mossy fibers appeared to reflect, temporally and topographically, the developmental gradients followed by their parent granule cells. The Golgi material confirmed the presence of mossy fibers in the hilus on the first postnatal day. Fasciculi of mossy fibers were observed in the stratum lucidum of the 3-day-old hippocampus, and although these immature axons were devoid of large synaptic expansions, they did have prominent growth cones at their termini. Small expansions along the lengths of the axons first appeared on day 7 and these grew to approximately an adult size and complexity by about day 14. The postsynaptic component of the mossy fiber synapse, the “thorny excrescence,” did not begin to emerge from the proximal portion of the pyramidal cell dendrites until sometime after day 9. At the electron microscopic level we observed, on the first postnatal day, small, immature mossy fiber expansions which made both symmetric and asymmetric contacts directly with dendritic shafts. These profiles, which were only one tenth the size of mature expansions, grew rapidly between postnatal days 1 and 9 and increased their mean area by a factor of five. On or about day 9, as the “thorny excrescences” emerged, the asymmetric synapses came to be associated with these spinous processes. Taken together, the Golgi and electron microscopic analyses support the suggestion that mossy fibers establish synaptic contact with pyramidal cell dendrites early in the postnatal period, several days before there is any indication of spine development. Furthermore, the “thorny excrescences” develop after the more typical, pedicellate spines have appeared on the distal pyramidal cell dendrites. Finally, while it is clear that the mossy fibers in our 21-day-old material are, for the most part, fully matured, a more subtle and protracted development of the system, long into adulthood, is indicated by the increased area and density of stained particles in the Timm preparations from adult animals.     

Roles of afadin in the formation of the cellular architecture of the mouse hippocampus and dentate gyrus

The hippocampal formation with tightly packed neurons, mainly at the dentate gyrus, CA3, CA2, and CA1 regions, constitutes a one-way neural circuit, which is associated with learning and memory. We previously showed that the cell adhesion molecules nectins and its binding protein afadin play roles in the formation of the mossy fiber synapses which are formed between the mossy fibers of the dentate gyrus granule cells and the dendrites of the CA3 pyramidal cells. We showed here that in the afadin-deficient hippocampal formation, the dentate gyrus granules cells and the CA3, CA2, and CA1 pyramidal cells were abnormally located; the mossy fiber trajectory was abnormally elongated; the CA3 pyramidal cells were abnormally differentiated; and the densities of the presynaptic boutons on the mossy fibers and the apical dendrites of the CA3 pyramidal cells were decreased. These results indicate that afadin plays roles not only in the formation of the mossy fiber synapses but also in the formation of the cellular architecture of the hippocampus and the dentate gyrus.     

Residual granule cells can maintain susceptibility of CA3 pyramidal cells to kainate-induced epileptiform discharges

Slices of adult rat hippocampus made from animals exposed neonatally to X-ray irradiation were studied with electrophysiological techniques. A single dose of 6 Gy irradiation of the pup's head significantly but unevenly reduced the number of granule cells in the dentate gyrus. A larger reduction was detected in the septal than in the temporal hippocampus. The number of hilar cells decreased also. Effects of irradiation were confirmed with histological techniques. Field potential responses to mossy fiber stimulation in the pyramidal layer of the CA3 subfield was smaller in irradiated than in normal rats. Superfusion of the slices with kainic acid (KA, 300–500 nM ) induced spontaneously recurrent paroxysmal activity (SRPA) in about 40% of irradiated slices in contrast with nearly 90% of slices cut from nonirradiated rats. Intracellular recordings from CA3 pyramidal cells in irradiated rats revealed recurrent bursts of action potentials on top of large depolarizing waves after KA application. Cells impaled in slices from the septal half of hippocampus of irradiated rats failed more often to respond with bursts to KA than cells in slices cut from the temporal half. Removal of mossy fiber input can therefore reduce KA induced hyperexcitability of CA3 pyramidal cells, but quantitative factors such as proportional loss of granule and hilar cells may explain the considerable differences found among cells and slices. Removal of 80% of granule cells reduces hyperexcitability consistently, while SRPA can be found in slices where as much as 50% of granule cells are missing. Intracellular findings suggest that failures of detection of SRPA following KA application to hippocampal slices of irradiated rats does not necessarily mean that CA3 pyramidal cells are no longer responding to KA with epileptiform bursting. Hippocampus 1998;8:548–561. © 1998 Wiley-Liss, Inc.     

Immunocytochemical localization of glutaminase-like and aspartate aminotransferase-like immunoreactivities in the rat and guinea pig hippocampus

There is considerable evidence that pathways of the hippocampus use an excitatory amino acids as transmitter. We have attempted to immunocytochemically identify excitatory amino acid neurons in the hippocampus of the rat and guinea pig using antiserum to glutaminase and antiserum to aspartate aminotransferase, which have been proposed as markers for aspartergic/glutamergic neurons. Glutaminase-like immunoreactivity was seen in granule cells in the dentate gyrus and fibers and puncta associated with the mossy fiber pathway in the hilus and stratum lucidum of the hippocampus. At the ultrastructural level, glutaminase-like immunoreactivity was observed in mossy fiber terminals in the stratum lucidum. Glutaminase-like immunoreactivity was also seen in pyramidal cells in regio inferior and regio superior and in cells in layer two of the entorhinal cortex. Schaffer collateral terminals, commissural fiber terminals and perforant pathway terminals were not seen at the light microscopic level. Glutaminase-like immunoreactivity is thus found in the cell bodies of proposed excitatory amino acid neurons of hippocampal pathways, but does not appear to label all terminals. Aspartate aminotransferase-like immunoreactivity was not seen in any cells, fibers or terminals in the rat or guinea pig hippocampus.     

Connections of the hippocampal formation in humans: I. The mossy fiber pathway

The hippocampal formation has been one of the most extensively studied cortical regions in rats, yet little is known about the anatomical connections of the hippocampus in primates, especially humans. With the use of an antibody against the calcium-binding protein, calbindin-D28K, in normal autopsy tissue and the neuronal tracers biocytin or biotinylated dextrans in in vitro slice preparations from tissue removed during surgery for intractable epilepsy, we examined the human hippocampal mossy fiber pathway. The injections of biocytin into the dentate granule cell layer labeled neurons in a Golgi-like manner, revealing the presence of basal dendrites on about 30% of the granule cells. The granule cell axons, the mossy fibers, initially formed a diffuse plexus of fibers in the polymorphic layer before organizing into fiber fascicles in the hilar pyramidal region. These fiber fascicles were much more prominent rostrally than caudally. Within the hilus and proximal portions of the extrahilar CA3 field, the mossy fibers ran through the pyramidal cell layer, and while near the transition to field CA2, the fibers turned superficially and crossed the pyramidal layer to run in the stratum lucidum. All of these features, seen following injections of tracer into hippocampal slices from the brains of epileptics, were confirmed by calbindin-staining of mossy fibers in normal brains. Biocytin-labeled mossy fiber axons revealed two characteristic types of enlargements: small varicosities and larger expansions. The expansions were found throughout the neuropil and were highly irregular, diaminobenzidine-dense profiles that had pleiomorphic modes of attachment to the parent axon. Electron microscopic images of these biocytin labeled expansions revealed that they were large synaptic boutons bearing asymmetric synapses. This study indicates that the human mossy fiber pathway shows some minor deviations from the rodent brain but little difference from monkeys. We argue that these changes mirror a phylogenetic growth of the CA3 pyramidal neurons (subfield CA3c) into the hilus rather than an evolutionary change of the mossy fiber pathway. This growth of subfield CA3c and the increase in mossy fibers running through the pyramidal layer (and a presumed accompanying increase in proximal basal dendritic contacts) may reflect a growing role of the projection from the dentate granule cells to subfield CA3c and from there to field CA1 in the primate hippocampus. J. Comp. Neurol. 385:325–351, 1997. © 1997 Wiley-Liss, Inc.     

The organization and development of the hippocampal mossy fiber system

The anatomical organization and development of the hippocampal mossy fiber system has been reviewed with special reference to its organization in the common laboratory rat. The mossy fibers originate from the granule cells of the dentate granular layer and the few granule cells found scattered in the dentate molecular layer and hilus. Via a complex system of collaterals the mossy fibers terminate on several types of neurons in the hilus, e.g. the basket cells and the mossy cells. Upon leaving the hilus to pass into Ammon's horn, the mossy fibers converge to form a distinct band of fibers that terminates on the proximal part of the apical and basal dendrites of the pyramidal and basket cells of the regio inferior. In some mammalian species the mossy fibers may continue into the adjacent part of the regio superior. Despite differences in the number of granule cells and pyramidal cells at different septotemporal levels this organization is relatively uniform along the septotemporal extent of the hippocampus. During development the mossy fibers grow out in a sequential manner that matches the pattern of neurogenesis and the aggregation of the cells of origin. From the level at which they originate, the fibers diverge along the septotemporal axis in such a way that the oldest granule cells have the most extensive projections. The adult topographic organization, which is already apparent at the earliest developmental stages, is thus formed in a stepwise fashion. It is concluded that the organization of the hippocampal mossy fibers indicates that neuronal specificity should not be explained by cellular recognition alone, but rather as the cumulated product of the preceding sequence of developmental events that include neurogenesis, migration, aggregation and directed axonal outgrowth.     

The organization and development of the hippocampal mossy fiber system

The anatomical organization and development of the hippocampal mossy fiber system has been reviewed with special reference to its organization in the common laboratory rat.The mossy fibers originate from the granule cells of the dentate granular layer and the few granule cells found scattered in the dentate molecular layer and hilus. Via a complex system of collaterals the mossy fibers terminate on several types of neurons in the hilus, e.g. the basket cells and the mossy cells. Upon leaving the hilus to pass into Ammon's horn, the mossy fibers converge to form a distinct band of fibers that terminates on the proximal part of the apical and basal dendrites of the pyramidal and basket cells of the regio inferior. In some mammalian species the mossy fibers may continue into the adjacent part of the regio superior. Despite differences in the number of granule cells and pyramidal cells at different septotemporal levels this organization is relatively uniform along the septotemporal extent of the hippocampus.During development the mossy fibers grow out in a sequential manner that matches the pattern of neurogenesis and the aggregation of the cells of origin. From the level at which they originate, the fibers diverge along the septotemporal axis in such a way that the oldest granule cells have the most extensive projections. The adult topographic organization, which is already apparent at the earliest developmental stages, is thus formed in a stepwise fashion. It is concluded that the organization of the hippocampal mossy fibers indicates that neuronal specificity should not be explained by cellular recognition alone, but rather as the cumulated product of the preceding sequence of developmental events that include neurogenesis, migration, aggregation and directed axonal outgrowth.     

Sprouting of GABAergic and mossy fiber axons in dentate gyrus following intrahippocampal kainate in the rat.

The present study examined the bilateral synaptic rearrangements of presumed gamma-aminobutyric acid (GABAergic) inhibitory axons and mossy fiber (presumed excitatory) recurrent collaterals following intrahippocampal kainic acid (KA) injection. Glutamate decarboxylase immunoreactivity (GAD-IR) was used to study inhibitory axon terminal sprouting, following 0.5 microgram KA/0.2 microliter injected unilaterally into the posterior hippocampus of rats (n = 16), with survival periods of 14, 28, and 120 days. The age-matched control animals (n = 9) received intrahippocampal 0.2 microliter saline (sham, n = 4) or no injection (normal, n = 5). To study mossy fiber synaptic rearrangements, 0.5 microgram KA/0.2 microliter volumes were injected unilaterally into the posterior hippocampus of rats (n = 10), with survival periods from 14, 28, and 120 days, and Timm sulfide-stained tissue sections were compared to age-matched sham (n = 4) or normal controls (n = 4). At 14 through 120 days after posterior KA injection, GAD-IR puncta were significantly increased in the ipsi- and contralateral inner molecular layers (IML) of the fascia dentata (FD) when compared to sham or normal controls. KA lesion also induced mossy fiber recurrent collateral sprouting into the ipsi- and contralateral FD IMLs. The loss of both the commissural and ipsilateral associational afferents to the FD apparently induced sprouting into their ipsi- and contralateral termination zones by granule cell mossy fibers and GAD-IR axons, thus establishing an abnormal circuitry near the observed pathology in the kainate model of epilepsy. Although reactive synaptogenesis of mossy fibers producing monosynaptic excitation may be one mechanism for KA epileptogenicity, the concurrent sprouting of GABAergic terminals in the same IML zone of the FD suggests that anomalous inhibitory synapses may contribute to chronic KA hippocampal hyperexcitability.