Dentate Cell Research Articles

Emx2Is Required for Growth of the Hippocampus But Not for Hippocampal Field Specification

The vertebrate Emx genes are expressed in a nested pattern in early embryonic cerebral cortex, such that a medial strip of cortex expresses Emx2 but not Emx1. This pattern suggests that Emx genes could play a role in specifying different areas or fields of the cortex along the mediolateral axis. Such a role has been supported by the observation that in mice lacking functional Emx2 the hippocampus is shrunken and the most medial field of the cortex, the hippocampal dentate gyrus, appears by cytoarchitecture to be missing (Pellegrini et al., 1996; Yoshida et al., 1997). Use of region-specific molecular markers shows, however, that hippocampal fields are specified and correctly positioned in the Emx2 mutant. In particular, a dentate cell population is generated, although it fails to form a morphological gyrus. This failure may be part of a more widespread medial cortical defect in the mutant. Examination of cortical cell proliferation and differentiation indicates a disruption of the maturation of the medial cortex in the absence of Emx2. Thus, Emx2 is required for normal growth and maturation of the hippocampus but not for the specification of cells to particular hippocampal field identities.

Ultrastructure of pits inPinus banksiana lamb

The cross-sectional view of pitting between various cell types inPinus banksiana Lamb. was studied at the ultrastructural level. Cell types inPinus banksiana include longitudinal tracheids, ray tracheids, ray parenchyma cells, “buffer cells” and epithelial cells. Two common characteristic features of bordered pit-pairs between longitudinal tracheids are an initial pit border and a thickened torus at the center of the pit membrane. The shape and size of the pit border and torus of bordered pit-pairs between two compression wood cells, and between the last-formed latewood longitudinal tracheid and first-formed earlywood longitudinal tracheid were different from those in the earlywood and latewood longitudinal tracheids. The pit border on the ray tracheid side varied in size and shape due to wall dentation. No initial pit border was found on the pit border of the ray tracheid side. The shape of bordered pit-pairs between two ray tracheids varied considerably due to irregularity of the dentate cell wall. The size of bordered pit-pairs in longitudinal tracheids was between 16 μm to 20 μm, which was twice the diameter of bordered pit-pairs in ray tracheids. Bordered pitpairs at the end wall of two ray tracheids appeared to be the smallest at 5 μm, Pit aspiration occurred in the bordered pit-pairs with or without a torus. In the heartwood zone, some half-borders pit-pairs between tracheary and ray parenchyma cells showed an additional secondary wall on the ray parenchyma cell side. Plasmodesmata were found in the half-bordered pit-pairs as well in the simple pit-pairs. Blind pits were observed between a ray tracheid and a longitudinal tracheid. Bordered pit-pairs between two “buffer cells” were also observed. The possible functions of buffer cells were discussed.

Ensheathing cells: Large scale purification from adult olfactory bulb, freeze-preservation and migration of transplanted cells in adult brain

Regenerating nerve fiber sprouts enveloped by olfactory bulb (OB) ensheathing cells (ECs) seem to escape the inhibitory influence of gliotic tissue. Accordingly, these cells may be useful for general repair of injured CNS. Relatively large numbers of ECs could be purified from confluent cultures of adult rat olfactory bulb using immunomagnetic beads. Viable ECs could be cultured and purified in good yield from OB dissected up to 18 h post-mortem. Purified ECs could be stored frozen at -75°C for at least 6 months, while maintaining 95% viability. ECs labelled with the fluorescent cell-linker PKH-26 neither shed the label nor exchanged it with other cells. The migration of labelled ECs transplanted to adult hippocampus was examined at intervals ranging from 3 h to 30 days. Active migration from the injection site was first observed 4 days after transplantation, when ECs appeared intercalated between the neurons of the hippocampal and dentate cell layers. Some ECs remained in that location after 30 days but, at that time, the olfactory glial cells could be observed in loci as distant and diverse as the laterodorsal thalamic nuclei, internal capsule, arcuate nucleus, cerebral aqueduct walls and choroid plexus. ECs seemed to have preferences for the dentate hilus, the pyramidal and granular cell layers, choroid plexus, blood vessels and putative peptidergic loci.

Synchronization of area CA3 hippocampal pyramidal cells and non-granule cells of the dentate gyrus in bicuculline-treated rat hippocampal slices

A recent study has described synchronous burst discharges of dentate hilar neurons and area CA3 pyramidal cells in the presence of the convulsants 4-aminopyridine and picrotoxin in guinea-pig hippocampal slices [Müller W. and Misgeld U. (1991) J. Neurophysiol. 65, 141–147]. To examine the synchronous activity of dentate cells and area CA3 pyramidal cells further, epileptiform burst discharges were examined in morphologically and/or electrophysiologically identified non-granule cells in the hilus and granule cell layer of the rat dentate gyrus and compared to simultaneously-recorded pyramidal cells of area CA3a, b, and c. Specifically, the types of dentate cells and the types of discharge were examined, as well as the timing of burst discharge of dentate cells relative to different cells of area CA3. In the presence of the GABA A receptor antagonist bicuculline (30 μM), all dentate cell types discharged in rhythmic, spontaneous bursts that were synchronized with area CA3 pyramidal cell epileptiform bursts. The sampled cells included hilar “mossy” cells, hilar fast-spiking cells (putative interneurons) as well as interneurons located in the granule cell layer, such as the pyramidal “basket” cells. Simultaneous recording from dentate non-granule cells and area CA3 pyramidal cells during exposure to bicuculline demonstrated that stimulus-evoked and spontaneous epileptiform bursts occurred almost exactly at the same time; there were only a few milliseconds between the onsets of pyramidal cell bursts and dentate cell bursts, with the pyramidal cell preceding the dentate cell in almost every case. There were no systematic differences among dentate cell types in the extent they lagged behind pyramidal cells, and there were no detectable differences among area CA3 pyramidal cells. In slices that were cut between area CA3 and the dentate gyrus, epileptiform bursts occurred in area CA3 but not in the dentate. These findings suggest that, in the absence of GABA A receptor-mediated inhibition, excitatory pathways from area CA3 to the dentate gyrus are strong and widespread. These pathways, and possibly other mechanisms, can lead to tightly synchronized action potential discharge of pyramidal cells and dentate non-granule cells. The results also suggest that disinhibition alone is insufficient to cause synchronous bursts in the dentate gyrus, in contrast to area CA3.

Enhanced host perforant path innervation of neonatal dentate tissue after grafting to axon sparing, ibotenic acid lesions in adult rats.

This study examines to which extent developing dentate granule cells grafted into excitotoxic lesions of the adult rat fascia dentata can be appropriately innervated by the host brain. The lesions were induced by focal injections of ibotenic acid (IA) and resulted in localized dentate and hippocampal neuronal cell death, but sparing of the afferent connections, now deprived of their targets. One week later pieces of fascia dentata from newborn rats were grafted into the lesions. After 6 weeks to 9 months the recipient brains were processed and analyzed by cell stain, histochemistry, immunohistochemistry, anterograde nerve fiber degeneration methods, and electron microscopy. Dentate grafts survived well in the lesion area and became organo-typically organized. They contained the normal nerve cell types of the fascia dentata and hilus (CA4), including the peptidergic somatostatin-, cholecystokinin- and enkephalin-reactive ones. The grafts were innervated by AChE-positive, cholinergic fibers from the host septum, and perforant path fibers from the host entorhinal area. The presence of the latter were demonstrated by Timm staining and light and electron microscopy of anterograde axonal degeneration. When the extent and density of the host perforant path innervation was examined and mapped at the electron microscopical level the grafts in the IA-lesions were found to receive a more extensive and denser host innervation than grafts placed in the normal fascia dentata of adult rats without a preceding axon-sparing ibotenic acid lesion. In this way the results demonstrate that certain lesion types can enhance the innervation of intracerebral grafts by already mature neural pathways of the point-to-point type.

Intracellular records of carbachol-induced theta rhythm in hippocampal slices

Intracellular recordings were made in the CA1, CA3 and dentate cell layers prior to, during and after the bath perfusion of 50 μM carbachol on hippocampal slices. Fifty-six percent of the cells in this sample were termed theta (θ)-related, ie., they exhibited membrane potential oscillations of 5–28 mV and rhythmic spike discharges related to the carbachol-induced extracellular θ-rhythm. The remaining 44% of the cells did not show the above relationship to the extracellular θ-rhythm. Carbachol produced an overall depolarization in all cells, in the range of 10–20 mV. These results demonstrated the cellular basis of carbachol-induced θ in hippocampal slices. This preparation will be a valuable model for studying cellular mechanisms and network properties underlying electroencephalographic activity.