Mechanisms of axonal guidance. The problem of intersecting fiber systems

The extracellular matrix molecule, laminin, induces purkinje cell dendritic spine proliferation in granule cell depleted cerebellar cultures

Granule cells and glia were eliminated or reduced in organotypic cerebellar cultures exposed to cytosine arabinoside. Transplantation of such granuloprival cultures with glia or exposure to astrocyte conditioned medium in the absence of parallel fibers (granule cell axons) resulted in proliferation of Purkinje cell dendritic spines. The aim of the present study was to identify specific astrocyte secreted factors that induced dendritic spine proliferation. Known astrocyte secreted, neurite promoting factors were screened by application to granuloprival cultures and assayed for dendritic spine proliferation by electron microscopy. An extracellular matrix molecule, laminin, evoked sprouting of Purkinje cell dendritic spines. Dendritic spine proliferation was not associated with known neurite promoting parts of the laminin molecule, as two laminin-derived peptides with identified neurite promoting domains did not induce dendritic spine sprouting. The purpose of laminin-induced dendritic spine proliferation may be to elaborate postsynaptic membrane, thereby increasing the target area for arriving axon terminals during development or regeneration, both of which have been associated with the presence of laminin secreting astrocytes.

Neural plasticity in cerebellar cultures

Cerebellar granule cells and oligodendrocytes are destroyed and astrocytes are functionally compromised by exposure of organotypic cerebellar cultures derived from newborn mice to cytosine arabinoside for the first 5 days in vitro. Consequently, myelin does not form and Purkinje cells survive in increased numbers, but without astrocytic ensheathment. In the absence of glial sheaths, Purkinje cells have altered membrane properties and reduced input resistance. Their inhibitory recurrent axon collaterals sprout enormously and hyperinnervate the unensheathed somata of other Purkinje cells and form heterotypical synapses with Purkinje cell dendritic spines normally occupied by homotypical excitatory parallel fiber (granule cell axon) terminals. This reorganization of the cortical circuitry, in which recurrent axon collaterals are the dominant inhibitory elements, allows retention of some inhibition in the absence of parallel fiber excitation of the inhibitory interneurons. In the absence of neuronal activity, the full complement of inhibitory synapses is not developed and the cultures exhibit sustained cortical hyperactivity after recovery from the blockade. If granule cells and glia are replaced, a second round of reorganization ensues, in the direction of restoration of the normal cortical circuitry. The cultures are myelinated and the number of recurrent axon collaterals is reduced. Astrocytes ensheath Purkinje cell somata and strip excess axosomatic synapses, as well as eliminate some of the heterotypical synapses in the cortical neuropil. Parallel fibers synapse with already present Purkinje cell dendritic spines and with newly proliferated spines, the latter induced by an astrocyte secreted factor. As homotypical synapses develop and heterotypical synapses decline, Purkinje cells undergo apoptosis and their population is reduced to control levels. With the restoration of parallel fiber excitation, recurrent axon collaterals are no longer the dominant cortical inhibitory elements. If neuronal activity is blocked as the granule cells and glia are replaced, there is incomplete formation of inhibitory synapses, and cortical discharges are hyperactive after recovery from activity blockade.

Collagen implants and cortico-spinal axonal growth after mid-thoracic spinal cord lesion in the adult rat

We describe an experimental model to study regeneration of lesioned corticospinal tract (CST) fibers in the adult rat spinal cord. After transection of all CST fibers at mid-thoracic level the gap is grafted with a sterile, cell-free collagen matrix. Two methods of collagen-application are used: 1) injection of a fluid collagen solution into the lesioned area which self-assembles in situ and 2) implantation of a solid collagen gel. At 4 weeks post-implantation CST axons are anterogradely labelled with horseradish-peroxidase (HRP). The collagen implant is evaluated for ingrowth of CST axons. The histopathological reaction (gliotic response) around the lesion and within the matrix is also studied. After application of a fluid collagen solution into the lesion area HRP-labelled CST axons can be visualized within the implant. In addition, astroglial and reactive microglial cells invade the collagen-matrix. On the other hand, if collagen is implanted as an already self-assembled gel, no ingrowth of labelled CST axons nor of astroglial/reactive microglial cells is observed. Both methods of collagen-application result in a considerable reduction of the gliotic response as compared to the ungrafted animals. We conclude that the method of application of collagen (i.e., fluid or gel) considerably affects the response of lesioned CST axons. The application of a fluid collagen graft which in situ self-assembles is beneficial for the regrowth of lesioned CST axons in rat spinal cord. In this respect the formation of an astroglial scaffolding structure within the (fluid) collagen, probably due to optimal integration between host and graft, is very important. The inability of injured CST fibers to enter the solid collagen graft may be related to the absence of an astroglial scaffolding structure within the implant.

The organization of the embryonic 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 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.

Cell death in suboptic necrotic centers of chick embryo diencephalon and their topographic relationship with the earliest optic fiber fascicles

The structural features of suboptic necrotic centers (SONCs) in the floor of the chick embryo diencephalon were studied. These necrotic areas were observed lateral to the prospective zone of the optic chiasm through developmental stages 14 to 24. The relationship between SONCs and the earliest optic fiber fascicles also was studied in an attempt to determine the possible significance of these cell death areas in the mechanism of optic pathway development. In SONCs, healthy neuroepithelial cells contain primary lysosomes and phagocytose fragments of dead cells. Discrete regions within the cytoplasm of some cells show electron-transparent vacuoles in contact with dense contents of ruptured lytic bodies. The cytoplasm of dying cells and dead cell fragments are notably electron dense, with numerous secondary lysosomes and electron-transparent vacuoles. These observations are interpreted on the assumption that after autophagic processes, condensation and fragmentation take place in dying cells of the SONCs. In the ventricular lumen adjacent to the SONCs, numerous more or less spherical bodies are observed that appear to be shed from the tip of the cells constituting the SONCs. Three different types of intraventricular bodies can be distinguished: loose, moderately dense, and highly dense. The first type appears to originate from apical portions of cells that undergo autolytic processes. Moderately dense fragments are interpreted as originating from dying cells in which the cytoplasm is undergoing condensation. Finally, highly dense intraventricular bodies appear to be fragments of dead cells that are shed into the ventricular lumen. SONCs separate the prospective area of the optic chiasm from lateral regions of the diencephalic floor. Extracellular spaces are poorly developed within the wall of the SONCs, whereas the neuroepithelium of the presumptive optic chiasm and regions located rostral and caudal to SONCs show abundant and extensive extracellular spaces. These are bounded by long marginal processes of neuroepithelial cells. Sagittal sections of embryonic heads at stages 22-24 reveal optic fiber fascicles penetrating the SONCs asymmetrically, as they are found only in its caudal half. These observations suggest that the SONCs function as doorways made of compact neuroepithelium, to be traversed by the earliest optic fibers before they reach the middle zone of the floor of the diencephalon through which they travel to the contralateral optic tract within large extracellular spaces.

Development of intersecting CNS fiber tracts: the corpus callosum and its perforating fiber pathway

What are the mechanisms acting during development at points of intersection of central nervous system fiber tracts which influence the direction taken by a population of growing axons? In order to address this question, the ontogeny of the intersecting rostral corpus callosum and its perforating fiber pathway (PF), and the microenvironment through which these fiber systems grow, were examined in a series of mouse embryos and early postnates. Our results show that the perforating fibers are identifiable in silver-stained sections between embryonic days (E) 15 and 16, at least 1 day prior to the initial appearance of the callosal projection. Soon after the PF can be identified, a dense accumulation of subventricular cells surrounds the PF at a point just ventral to the location where the callosum and PF will intersect (i.e., at the corticoseptal boundary). Callosal axons, which are present at the point of intersection beginning on E17, do not joint the perforating fibers, nor do they appear to penetrate the underlying population of subventricular cells. Instead, the callosal fibers turn across the PF and enter the contralateral cerebral hemisphere. Thus, the intersection of the callosal and perforating fiber systems during development may be related both to the sequential development of each pathway and to the altered nonneuronal environment at the point of intersection.

Death of the subcallosal glial sling is correlated with formation of the cavum septi pellucidi

In this study we have examined the developmental fate of a population of cells that is located beneath the rostral corpus callosum during the perinatal period. These cells form a distinct slinglike structure along the geographically defined corticoseptal boundary (CSB) and may play a role in guiding callosal axons across the midline. The sling is a transient structure present in fetal and neonatal animals but not in adults. Here we show that the CSB cells die and that this debris is removed by macrophages. The sequence of cell degeneration in the CSB is highly stereotyped and follows a spatiotemporal pattern that is correlated with fusion of the cerebral hemispheres and subsequent growth across the midline of the callosal axons. The subcallosal location of the resorbing CSB is found in the exact place in which a fluid-filled cavity (the cavum septi pellucidi) is transiently found during the perinatal period. The tight temporal and spatial correlation between callosal axon decussation, degeneration of the CSB, and cavum septi formation suggests that these three phenomena may be causally related.

Guidance of optic axons in vivo by a preformed adhesive pathway on neuroepithelial endfeet

Antibodies against the neural cell adhesion molecule (NCAM) were used in vivo both to localize NCAM antigenic determinants in developing tissues of the chicken visual system and to perturb cell-cell adhesion during growth of optic fibers to the tectum. The immunohistochemical studies revealed a staining pattern on neuroepithelial cells which coincided with certain regions of the presumptive route for optic axons, not only with respect to the overall pathway from the eye to the tectum, but also in the preferential distribution of the antigen on the marginal endfeet which are contacted by optic axon growth cones. The antibody-perturbation studies, which involved intraocular injection of anti-NCAM Fab at embryonic Day 3.5, demonstrated that inhibition of NCAM-mediated adhesion results in a dramatic distortion of growth cone-neuroepithelial cell relationships and consequently of the optic pathway. Together, these studies suggest that guidance of optic axons along the margin of the brain is at least in part influenced by a preformed adhesive pathway on neuroepithelial cells associated with NCAM antigens.