The time of origin of neurons in the hippocampal region of the rhesus monkey

The time of origin of neurons in the hippocampal region was determined in a series of rhesus monkeys, each of which had been exposed to a pulse of tritiated thymidine (³H-TdR) at a different time during ontogeny and sacrificed between the second and fifth month after birth. No heavily labeled cells were found in the hippocampal region of animals exposed to ³H-TdR before embryonic day 33 (E33). Exposure to ³H-TdR given at E36 labels a few neurons in the deepest layers of the entorhinal area, and ³H-TdR given at E38 labels a small number of neurons in all hippocampal subdivisions. Although the first neurons are generated almost simultaneously throughout the hippocampal region, the proliferation ceases at a different time in each subdivision. The last neurons destined for the entorhinal area and presubiculum are generated between E70 and E75, whereas the last parasubicular neurons are generated between E75 and E80. The production of neurons that form the subiculum ends about two weeks earlier, between E56 and E65. Within the hippocampus, genesis of pyramidal cells ends between E70 and E80 in area CA1, between E56 and E65 in area CA2, between E65 and E70 in area CA3, and between E75 and E80 in area CA4. In contrast, the genesis of granule cells of the fascia dentata is considerably prolonged. It continues throughout the second half of gestation, declines steadily in the course of the first postnatal month, and tapers off during the next 2 months. There is a distinct inside-to-outside spatiotemporal gradient in the parahippocampal formation and in the stratum pyramidale of both the subiculum and hippocampus. In contrast, the spatiotemporal pattern of granule cell origin in the dentate gyrus is outside-to-inside. Furthermore, granule cells generated between E36 and E80 are distributed in a distinct suprapyramidal-to-infrapyramidal gradient, whereas those generated at later ages are distributed evenly throughout the fascia dentata. Correlation of the present findings with histological data on hippocampal neurogenesis in the human brain demonstrates that the timing and sequence of developmental events as well as spatiotemporal gradients are similar in both primate species.     

Neurogenesis of glutamic acid decarboxylase immunoreactive cells in the hippocampus of the mouse. II: Area dentata

The temporal patterns of neurogenesis of cells showing glutamic acid decarboxylase (GAD) immunoreactivity were determined in the area dentata of the mouse. Pregnant C57Bl mice received pulse injections of (3H)thymidine from E11 through E17 (E0 being the day of mating). The distribution of (3H)thymidine-labeled, GAD-positive neurons in the hilus and in the different strata of the fascia dentata (stratum infragranulosum, stratum granulosum, stratum moleculare) were recorded in adult animals. A radial gradient of neurogenesis of GAD-positive cells in the area dentata was not apparent. In the transverse axis, neurogenesis of GAD-positive cells seemed to follow a faint suprapyramidal to infrapyramidal gradient, which was due to differential timing of neurogenesis of GAD-positive cells destined for the stratum infragranulosum of the suprapyramidal and infrapyramidal blades of the fascia dentata. GABAergic neurons in the fascia dentata comprise a limited number of well-defined cell types. All of the different morphologic types of GAD-positive neurons present in the area dentata were generated prenatally. These diverse forms did not have specific times of neurogenesis. These results support the concept that the adult morphology of GAD-positive cells in the area dentata of the mouse do not bear any relationship to their times of origin.     

The cells of origin of the commissural afferents to the area dentata in the mouse

The hippocampal commissural projection to the area dentata of the mouse was studied using the retrograde horseradish peroxidase (HRP) technique. Small volumes of HRP injected into the molecular layer of the fascia dentata or various subareas of regio inferior of the hippocampus (fields CA3a-c) resulted inlabeled perikarya in the contralateral hippocampus and area dentata. The commissural projection to the fascia dentata was observed to originate exclusively from cells within the hilus fasciae dentatae (CA4) of the contralateral area dentata. There was evidence of a considerable spread of commissural innervation along the septotemporal axis preferentially in the septal direction, confirming earlier observations. In contrast to the septotemporal spread, a sharp homotopic spatial organization was found in the mediolateral direction. For example, injections into the lateral portion of field CA3 (CA3a) resulted in HRP-positive cell bodies only in the contralateral field CA3a. When injections were made which apparently labeled all of the commissural fibers, the HRP reaction product was found in neurons both in the entire regio inferior and as far as the innermost point of the hilus fasciae dentatae; the majority of labeled cells were located in hippocampal subfield CA3c. No labeled cells were observed beyond the tip of the mossy fibers in regio superior.     

Hippocampal NPY neurons project to the fascia dentata in organotypic cultures

The distribution of neuropeptide Y immunoreactive (NPY-ir) neurons in organotypic cultures of hippocampi from neonates was compared to that seen in adult rats. In addition to the known NPY-ir neurons in the hippocampus proper and in the hilus of the fascia dentata, isolated, large, multipolar, NPY-ir neurons were observed in the subiculum and in areas CA1 and CA3. Their axons projected into stratum radiatum of the hippocampus proper and into the molecular layers and hilus of the fascia dentata where they branched profusely. These NPY-ir neurons were regularly distributed throughout the septo temporal extent of the hippocampus and were present in both neonates and adult hippocampi. The hilar NPY-ir neurons have always been considered the source of the NPY-ir plexus in the outer molecular layer of the dentate gyrus. However, our results show that there is also a contribution from the NPY-ir neurons in the hippocampus proper. © 1996 Wiley-Liss, Inc.     

Co-localization of somatostatin mRNA and parvalbumin in the dorsal rat hippocampus after cerebral ischemia

Following transient global ischemia most of the neurons containing somatostatin in the fascia dentata of the dorsal hippocampal formation die, while somatostatinergic neurons in the CA1 region survive. These neurons react to ischemia with a transiently reduced expression of somatostatin mRNA and peptide. We have tested the hypothesis that this selective vulnerability is solely related to those somatostatinergic neurons which do not express the calcium-binding protein parvalbumin. Postischemic changes were studied in rat dorsal hippocampus at 2 and 16 days after 10 min of global cerebral ischemia using a four-vessel occlusion model. We performed a double-staining visualizing the mRNA coding for somatostatin by non-radioactive in situ hybridization and parvalbumin protein by immunocytochemistry. Only 5% of the somatostatinergic cells in the fascia dentata contained parvalbumin. The number of somatostatinergic cells was permanently reduced following ischemia. Among surviving neurons we found cells with and without parvalbumin expression. Thus, expression of parvalbumin is not predictive for survival of somatostatinergic cells in the fascia dentata. In contrast, in CA1, 37% of the somatostatinergic cells contained parvalbumin. These cells were unaffected by the transient ischemic period. The somatostatinergic cells lacking parvalbumin showed transiently reduced mRNA levels at day 2, but recovered to control values at the 16th postischemic day. Thus, expression of the calcium-buffering protein parvalbumin coincides with resistance of somatostatinergic neurons in CA1 to transient effects of ischemia. We conclude that the calcium-buffering capacity of parvalbumin may partially contribute to the protection of somatostatinergic neurons from ischemia in the dorsal hippocampus. However, the survival of somatostatinergic cells without parvalbumin indicates the importance of other factors as well. © 1995 Wiley-Liss, Inc.     

Vascular patterns of the rat hippocampal formation

Little is known of the course and distribution of blood vessels supplying and draining the hippocampus. Such information could be of value in designing and evaluating lesion and ablation experiments and may reflect spatial properties of neurons. This study mapped the distribution of major arteries and veins of the rat hippocampal formation. Arteries and veins of adult female Wistar rats anesthetized with sodium pentobarbital were injected with silicone rubber. Double injections to demonstrate both arteries and veins in the same animal were with India ink and a fluorescent material. Arterial supply to the hippocampus was via transverse hippocampal arteries that stemmed from the longitudinal hippocampal artery, a branch of the posterior cerebral artery. Internal transverse hippocampal arteries located in the hippocampal fissure supplied small, short branches to the adjacent blade of the fascia dentata, part of the area dentata, and CA3 fields. Other branches of the longitudinal artery supplied the remaining blade of the fascia and area dentata, subicular fields, and entorhinal structures. Internal transverse hippocampal veins located in the hippocampal fissure alternated in position with the arteries and appeared to be paired with, and to drain fields supplied by, the internal transverse arteries. Deep transverse hippocampal veins, unaccompanied by arteries, received branches in the intraventricular alveus and adjacent stratum oriens of CA3. The transverse veins drained into longitudinal vessels or the basal vein. Although transversely directed arteries and veins may suggest a hippocampal lamellar neuronal organization, microvascular fields must be mapped before claims are made for a totally segmental vascular architecture in the hippocampus.     

Organization of the GABAergic system in the rat hippocampal formation: a quantitative immunocytochemical study

Specific antibodies against gamma aminobutyric acid (GABA) and glutamic acid decarboxylase (GAD) were used to study the organization of the GABAergic system in the rat hippocampal formation. Both the number of GABA-like-immunoreactive (Li) somata and neuropil density were assessed in semithin sections. Cell counts revealed that approximately 11% of the hippocampal neuronal population showed GABA-Li within the various planes of section. Each layer in the hippocampal formation had a characteristic organization of GABA-Li elements. In Ammon's horn, 80-95% of the neuronal somata within the apical and basal dendritic regions were GABA-Li positive. Within the pyramidal cell layer 5-8% of the cells were GABA-Li in the CA1 to CA3 subfields of Ammon's horn and only 3% were GABA-Li within that portion of the pyramidal cell layer that inserts into the hilus. Only slight differences in the density of the GABA-Li neuropil were observed within the CA1-CA3 dendritic regions. Restricted to the stratum lucidum was a dense band of GABA-Li label. Counts of immunoreactive grains localized on the perimeter of pyramidal (CA1-CA3) and granule somata revealed more terminal boutons on the CA3 cells than on CA1 and granule neuronal somata. A topographical distribution of GABA-Li somata and neuropil was found in the fascia dentata: There the label particularly concerned its suprapyramidal and rostrolateral portions. Approximately 40% of neurons in the molecular layer, 60% in the polymorph layer, and 18% within the hilar region were GABA-Li. Within the granule cell layer only 2% of the neurons were GABA-Li positive. Distinct differences in the density of the GABA-Li neuropil were present in the molecular, pericellular granular, and hilar regions of the fascia dentata. While the morphology of GABA-Li neuronal somata varied according to their hippocampal layer, the most heterogeneous cell types were found in the regio inferior of the hippocampus. There we have identified neurons that are reminiscent of the inferior region interneuron described in Golgi material by Amaral and Woodward (Brain Res. 124:225-236, '77). Moreover, particularly in the sagittal plane, we have identified oval, triangular, and round cells and that have processes oriented in a parallel arrangement, appearing to be aligned along the granule cell mossy fibers.     

Sparse, environmentally selective expression of Arc RNA in the upper blade of the rodent fascia dentata by brief spatial experience

After a spatial behavioral experience, hippocampal CA1 pyramidal cells express the activity-regulated, immediate early gene Arc in an environment-specific manner, and in similar proportions ( 40%) to cells exhibiting electrophysiologically recorded place fields under similar conditions. Theoretical accounts of the function of the fascia dentata suggest that it plays a role in pattern separation during encoding. The hypothesis that the dentate gyrus (DG) uses a sparse, and thus more orthogonal, coding scheme has been supported by the observation that, while granule cells do exhibit place fields, most are silent in a given environment. To quantify the degree of sparsity of DG coding and its corresponding ability to generate distinct environmental representations, behaviorally induced Arc expression was assessed using in situ hybridization coupled with confocal microscopy. The proportion of Arc(+) cells in the "upper blade" of the fascia dentata (i.e., the portion that abuts CA1) increased in an environment-specific fashion, approximately 4-fold above cage-control activity, after behavioral exploration. Surprisingly, cells in the lower blade of the fascia dentata, which are capable of expressing Arc following electrical stimulation, exhibited virtually no behaviorally-induced Arc expression. This difference was confirmed using "line scan" analyses, which also revealed no patterns or gradients of activity along the upper blade of the DG. The expression of Arc in the upper blade was quantitatively similar after exploring familiar or novel environments. When animals explored two different environments, separated by 20 min, a new group of cells responded to the second environment, whereas two separated experiences in the same environment did not activate a new set of granular cells. Thus, granule cells generate distinct codes for different environments. These findings suggest differential contribution of upper and lower blade neurons to plastic networks and confirm the hypothesis that the DG uses sparse coding that may facilitate orthogonalization of information.     

Transient forebrain ischemia-induced neuronal degeneration in fascia dentata transplants

Fascia dentata tissue blocks from newborn rats were grafted into one-week-old, ibotenic acid-induced lesions of the fascia dentata, or the normal fascia dentata of adult rats. After at least 2 months survival the recipient rats were subjected to 10 min of forebrain ischemia (4-vessel occlusion), and examined 2 or 4 days later for neuronal degeneration in the host hippocampi and the transplants, by silver staining and immunohistochemistry. Transplants survived well in both normal and lesioned host brains, with easily recognizable subfields and layers and presence of normal types of principal and non-principal neurons. As expected, argyrophilic, degenerating neurons were present in the pyramidal cell layer of CAl and CA3c of the non-grafted contralateral host hippocampus and in the contralateral dentate hilus (CA4). In the hilus the degeneration corresponded to the loss of somatostatin-immunoreactive neurons, while parvalbumin-immunoreactive neurons were spared. In the dentate transplants degenerating neurons were observed in the granule cell layer, the hilus and the adjacent CA3 pyramidal cell layer. There was no obvious loss of either somatostatin- or parvalbumin-immunoreactive neurons. The degeneration varied considerably between transplants, from a few to large groups of silver stained neurons, but this difference did not display any obvious relation to grafting into normal or lesioned hosts, the exact location of the grafts or the general organization and distribution of intrinsic or extrinsic host afferents in the grafts. The results demonstrate that both ischemia-susceptible and -resistant types of neurons grafted to normal and lesioned adult rat brains are susceptible to transient forebrain ischemia after transplantation. In spite of an extensive reorganization of transplant nerve connections, the physiologicalbiochemical mechanisms necessary for the induction of ischemic cell death were accordingly present in the transplants.     

Spatial features of the rat hippocampal vascular system

Earlier investigations in this laboratory demonstrated paired internal transverse arteries and veins associated with the rat cornu Ammonis (CA), mapped the vessels, and suggested segmental distribution and drainage patterns. Further studies of the smaller branches and capillaries were required to resolve the presence or absence of structurally segmented or isolated capillary beds common to vertebrate forms having paired intramedullary arteries and veins. Adult Wistar rats were injected with inks to demonstrate the arterial or venous tree or both. Each of the CA3 internal transverse arteries usually supplies numerous small-diameter branches to the adjacent blade of the fascia dentata. Often a major ramus supplies CA1 before the artery branches most profusely near the deepest point of the hippocampal fissure to supply the extrahilar CA3 and portions of the area dentata. Other vessels are located in the subiculum, adjacent regions of CA1, and the area dentata. Branches of the CA3 vessels more nearly parallel the lateral hippocampal axis than either of the other two. Numerous microvascular rami distribute obliquely across the longitudinal and transverse axes. As readily observed from thick, cleared India ink tissue sections, the strata molecularelacumosum and oriens are more vascular than the stratum radiatum where the long axis of capillaries tends to be oriented parallel to the apical dendrites. Clearly a subzone of CA1 stratum pyramidale, but not CA3, is the least vascular CA region. The outer portion of stratum moleculare of the fascia dentata is more vascular than the inner third. Well-developed capillary plexuses exist adjacent to the vascular poor zone of stratum granulosum and CA1. Branches of the internal transverse veins, deep veins, and external veins drain CA3 fields. Evidence of nonexclusive pairing of the internal arteries and veins, multiple systems draining the CA3 field, and the presence of anastomosing capillary beds indicate that structurally isolated segmental microvascular organizations do not exist for CA3 of the rat hippocampus.     

Immunohistochemical localization of GDNF in the human hippocampal formation from prenatal life to adulthood

In this study, we examined the immunohistochemical occurrence and distribution of glial cell line-derived neurotrophic factor (GDNF) in autoptic specimens of normal human hippocampus at different ages, from 22 weeks of gestation (w.g.) to adult life. Two different anti-GDNF polyclonal antibodies were used. Western blot analysis on homogenates of human and rat brain and recombinant human GDNF resulted in differential detection of monomeric and dimeric forms of the proteins. The ABC immunohistochemical technique revealed that in the Ammon's horn, numerous positive cell bodies occurred in the pyramidal layer, the majority of them being present in the proximal CA1 and in CA2. Sparse positive neurons could be observed in the stratum oriens and moleculare. In the fascia dentata many granule cells showed a light punctate staining, whereas more heavily labelled neurons occurred in the polymorphic layer and, occasionally, in the molecular layer. The distribution pattern of GDNF-like immunoreactivity appeared consistently similar throughout life stages from 29 w.g. to adult age. However, intensity of labelling and frequency of neuronal cell bodies was highest in the neonate and decreased in adulthood. The present data provide a comprehensive map of the localization of GDNF-like immunoreactive neurons in the human archicortex at developmental ages and in the mature tissue and represent a first step towards the identification of hippocampal neurons which express the protein and/or are responsive to it. They further suggest that GDNF may play a role in the development of intrahippocampal circuitry and in neuronal function and maintenance throughout life.     

Origin of the synchronized network activity in the rabbit developing hippocampus

Rhythmic spontaneous bursting is a fundamental hallmark of the immature hippocampal activity recorded in vitro. These bursts or giant depolarizing potentials (GDPs) are GABA- and glutamatergic-driven events. The mechanisms of GDPs generation are still controversial, since although a hilar origin has been suggested, GDPs were also recorded from isolated CA3 area. Here, we have investigated the origin of GDPs in hippocampal slices from newborn rabbits. Simultaneous intracellular recordings were performed in CA3, CA1 and the fascia dentata. We found a high degree of correlation between the spontaneous GDPs present in CA3 and CA1 regions. Cross-correlation analysis demonstrated that CA3 firing precedes CA1 by about 192 ms, although a significant population of discharges was recorded first in CA1 (20%). Granule cells (GCs) in the fascia dentata also showed GDPs. The frequency of these events (1.46 ± 1.25 GDPs/min, n = 7) is significantly lower when compared with that from CA3 (3.13 ± 1.43 GDPs/min, n = 10) or CA1 (2.94 ± 1.36 GDPs/min, n = 17). Dual recordings from CA3 and fascia dentata cells showed synchronous bursts in both regions with no prevalent preceding area. By recording from isolated areas we found that CA1, CA3 and the fascia dentata can produce GDPs, suggesting that they emerge as a property of local circuits present throughout the hippocampus.     

Neurogenesis of glutamic acid decarboxylase immunoreactive cells in the hippocampus of the mouse. I: Regio superior and regio inferior

The neurogenetic gradients of neurons showing glutamic acid decarboxylase (GAD) immunoreactivity were determined in the regio superior and in the regio inferior of the mouse hippocampus. Pregnant C57Bl mice received pulse injections of (3H)thymidine from E11 through E17 (E0 being the day of mating). Distributions of (3H)thymidine-labeled, GAD-positive neurons in the different strata of the hippocampus proper were recorded in adult animals. GAD-positive neurons in this region are generated prenatally. Radial gradients of neurogenesis of GAD-positive cells are characterized by two main features: 1) with the exception of the stratum lacunosum-moleculare and its interface with the stratum radiatum, GAD-positive neurons of the plexiform strata are generated before those destined for the pyramidal layer; 2) within the pyramidal layer, GAD-positive cells are positioned according to an inside-out sequence. In the transverse axis, neurogenesis of GAD-positive cells follows a regio inferior to regio superior gradient. This gradient is due to prolonged neurogenesis of GAD-positive cells for the pyramidal layer in the regio superior. Given the selective laminar disposition of the GABAergic interneurons in the hippocampus, the present authors explored whether or not the diverse types of these interneurons could have specific birth dates and concluded that no relationship exists between birth dates and adult phenotypes of GAD-immunoreactive cells in the mouse hippocampus proper.     

GABAergic nonpyramidal neurons in intracerebral transplants of the rat hippocampus and fascia dentata: a combined light and electron microscopic immunocytochemical study

Glutamate decarboxylase (GAD) immunocytochemistry was used to study GABAergic neurons and synapses in intracerebral allografts of the rat hippocampus and fascia dentata. Tissue blocks of regio inferior of Ammon's horn (hippocampal field CA3) or of the fascia dentata were taken from newborn rats and transplanted to the hippocampal region of young adult rats. After 6 1/2 months' survival the recipient brains were fixed by perfusion and serially sectioned on a Vibratome. Sections containing the transplant and/or the host hippocampal region were immunostained for GAD and flat-embedded in Araldite for a correlated light and electron microscopic analysis. Immunostained neurons and terminals in the transplants were compared to immunoreactive elements in the hippocampus and fascia dentata of the hosts and other, normal rats. As in the hippocampal formation in situ, GAD-immunoreactive neurons and terminals in the transplants were observed in all layers. In dentate transplants a preponderance of immunostained cells was found just beneath the granule cell layer. In both hippocampal and dentate transplants, immunoreactive terminals were most abundant in the cell layers where they formed characteristic pericellular baskets around the pyramidal and granule cell bodies. In the electron microscope, the transplant GAD-immunoreactive neurons exhibited numerous cytoplasmic organelles, deeply infolded nuclei, and nuclear rods. Immunoreactive terminals formed symmetric synaptic contacts on the cell bodies, dendritic shafts, and spines of transplant pyramidal cells, granule cells, and hilar neurons. These are normal characteristics of GAD-immunoreactive neurons and terminals as also observed in the hippocampus of the host rats and the normal controls. Our results demonstrate that GABAergic neurons survive transplantation and develop a cell-specific morphology that includes the axonal projections.     

Distribution of glutamate-decarboxylase-immunoreactive neurons and synapses in the rat and monkey hippocampus: light and electron microscopy

We have studied the distribution of gamma-aminobutyric acid (GABA) neurons, axons, and synapses in the rat and monkey hippocampal formation by using glutamate decarboxylase (GAD) immunocytochemistry together with Nissl stains, electron microscopy, and double-labeled retrograde transport of horseradish peroxidase. The numbers of GAD-containing (putative GABA) neurons and their percentages compared to all Nissl-stained neurons were calculated throughout all the various fields and strata of the mammalian hippocampus. Although their numbers are greatest in the polymorph region of the fascia dentata (FD) and in the principal cell layers stratum pyramidale (SP) and stratum granulosum (SG), GAD immunoreactive (GAD-IR) cells are numerous in other strata that contain mostly dendrites and scattered cells. These GAD-IR (putative GABA) neurons in dendritic regions may be involved in feedforward dendritic inhibition or may directly inhibit nearby neurons. We used a postmortem delay technique, which resulted in apparent diffusion of GAD into dendrites and axons and allowed better visualization of the extensive dendritic domain of GAD-IR neurons. Computerized image analysis of GAD-IR puncta indicated that putative GABA terminals were numerous on apical and basilar dendrites of all pyramidal cells but unexpectedly highest in the monkey presubiculum. In the rat, GAD-IR neurons projected axons ipsilaterally from every region to the fascia dentata and CA1; however, commissural GAD-IR axons to the fascia dentata arose from GAD-IR neurons in only the contralateral fascia dentata and subiculum. Electron microscopy of GAD-stained hippocampus identified GAD-IR neurons with non-GAD-IR (possibly excitatory) synapses and GAD-IR terminals on somata and dendrites, 80% being the symmetric type and 20% the asymmetric type. In contrast, non-GAD-IR terminals were asymmetric 80% of the time.     

Fine structure of identified neurons in the primate hippocampus: a combined Golgi/EM study in the baboon

The hippocampi of two 1-year-old female baboons (Papio anubis) were used for a combined Golgi/electron microscope (EM) study of characteristic cell types in the hippocampus proper and fascia dentata. Results were compared with previous Golgi/EM studies of hippocampal neurons in small laboratory animals. Cell bodies of pyramidal neurons in CA1 were more loosely distributed than known from studies on the rat or guinea pig. Numerous basal and horizontal dendrites originating from the perikaryon filled in the space between neighboring cell bodies. Apical stem dendrites were varying in length, depending on the position of the parent cell body in outer or inner portions of the pyramidal layer. Dendrites were densely covered with spines which in the EM showed very complex synaptic contacts. In contrast to our observations in rats and guinea pigs, CA3 pyramidal cells in the monkey hippocampus exhibited numerous large spines or excrescences not only on apical dendrites but also on basal dendrites running through stratum oriens. These excrescences appeared to be more complex than in small rodents. They often branched, protruding deeply into presynaptic mossy fiber boutons, and formed multiple asymmetric synaptic contacts. Granule cells of the monkey fascia dentata, in contrast to those of the rodent, occasionally had basal dendrites extending into the hilar region. In the EM, granule cells either with or without basal dendrites exhibited fine structural characteristics that were very similar to those described in Golgi/EM studies of granule cells in the rat fascia dentata. Of the various types of nonpyramidal neurons the horizontal cells in stratum oriens with dendrites parallel to the alveus were analyzed. As seen in rats, these cells exhibited large amounts of rough endoplasmic reticulum, indentations of the nuclear membrane, and nuclear inclusions. Numerous terminals formed synaptic contacts on dendritic shafts. In contrast to rodents, numerous spines arose from dendrites and cell bodies of these neurons. In the EM, often single spines were found to establish synaptic contacts with several presynaptic boutons. In summary, our correlated light and EM study of four characteristic cell types, which are present in both nonprimates and primates, demonstrates a much more complex dendritic pattern and synaptic organization of these neurons in primates than in commonly studied small laboratory animals.     

Cellular, histochemical and connective organization of the hippocampus and fascia dentata transplanted to different regions of immature and adult rat brains

The aims of the present study were to examine the survival and the cellular and connective differentiation of intracerebral transplants of fascia dentata and hippocampus. Pieces of immature dentate and hippocampal tissue were taken from late embryonic (E18) and early postnatal (1-9 days old) rats and transplanted into the brains of 1- to 13-day-old and adult rats. After survival times from 4 days to 2 years the cellular and connective organization of the transplants was monitored in parallel series of sections stained with thionin (cell bodies), Timm's sulphide silver method (terminal fields). Nauta and Fink-Heimer methods (normal and degenerating fibers) and a method for AChE activity (cholinergic afferents). The transplants survived well in all combinations of donor and recipient ages used, and they survived and differentiated in all parts of the recipient brains, although relations to pial and ventricular surfaces appeared to be optimal. Cell differentiation continued after transplantation, and a characteristic laminar organization was retained, although least in embryonic donor tissues. The distribution of intrinsic connections was determined by the types of subfields present in the transplants and interaction with ingrown host afferents. All aberrant intrinsic connections observed corresponded to aberrant connections formed in the hippocampus and fascia dentata denervated in situ and included supragranular mossy fibers in the fascia dentata, aberrant infrapyramidal mossy fibers in CA3, spread of CA4-associated afferents beyond the normal commissural-associational zone in the dentate molecular layer together with ingrowth of CA3-associated and CA1-subiculum-associated afferents. Most transplants received a cholinergic input of host origin irrespective of the localization in the host brain, but also non-cholinergic host pathways innervated the transplants, in particular when the transplants were in close contact with host fiber tracts, and when the recipients were immature. At various transplant locations the non-cholinergic host afferents belonged to the commissural hippocampo-dentate system, the commissural hippocampal system and the callosal system. Other cases suggested innervation of dentate transplant by host entorhinal afferents. The formation and distribution of intrinsic transplant connections and connections between transplant and host appeared to be regulated by the same factors that regulate the development and reorganization of fiber connections in the normal and the in situ denervated hippocampus and fascia dentata. As a special variety of this, the distribution of cholinergic afferents adjusted to the distribution of the major intrinsic and extrinsic non-cholinergic pathways.     

Development of the cholinergic system in control and intra-uterine growth retarded rat brain

The activity of choline acetyltransferase (ChAT), acetylcholinesterase (AChE), and muscarinic receptors was studied in control rats and in rats growth-retarded in utero because of reduction of the blood supply 5 days before birth. The different markers of the cholinergic system were estimated at P (postnatal day) 6, 9, 12, 15, 22 and 60 in cerebellum, hypothalamus, septum, striatum and CA1, CA3 and fascia dentata of the hippocampus. In control rats, there was a transient increase in ChAT activity in the septum during the second week of postnatal development. In the intrauterine growth retarded rats there was a marked delay in this developmental rise in CA1, CA3 at P6 and P9 and in the fascia dentata at P14 respectively. This delayed rise enzyme activity was associated with a significant reduction of muscarinic binding sites [( 3H]QNB) in the hippocampus. AChE staining showed a similar development in both groups. Therefore, the undernutrition produced by a reduction of the blood supply 5 days before birth is associated with a delayed maturation of cholinergic functions.     

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

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 perchorate (DiI) in the entorhinal cortex, entorhinal axons were observed for the first time inthe 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. © 1994 Wiley-Liss, Inc.